I have seen a lot of misinformation about GOG Galaxy 2.0 in these forums, and even most of the basic functionalities are not even known.
This is pretty understandable because most of the time what you can do is not exactly where you will expect it in this client, but still that doesn't mean it is lacking functionalities.

This guide will show how to obtain an interface very similar to the old 1.2 client (without the need of complex modifications).

You can see a preview in the attached "finalresult.jpg", with the old library interface, a direct forum link and installed games listed to the left.

1) The Bookmark Flag (bookmarkflag.jpg picture attached)

This is a fundamental function of the client, and it's needed to obtain the results of all the following instructions.
It's an icon on the top of the window, to the right of the history arrows and can be ACTIVATED / DEACTIVATED, it will create a direct link to the page you are watching, that will appear in the left column of the client.
These links can then be reordered with the mouse, and also renamed with a right click.

From now on I will refer to this function with the words "Bookmark this".

2) The Forum Link

The old client had a direct link to the GOG Forum.
To obtain the same here you'll have to follow these steps:

1. Select a game from the library to enter it's page.
2. Open the game options menu, it's the icon to the right of the PLAY button.
3. Select "View in Store" (warning DO NOT choose "View game forum")
4. From the game store page click "Forum discussion" (in the Game details section)
5. Now you are in that specific game forum page, so click on "Community" at the top of the page to access the main forum page.
6. Bookmark This !

From now on to access the forums you'll just have to click the link in the left column of the client.

(You can Bookmark every specific page of the forum you want to have in the list, let's say you are playing a game and have the need to constantly access it's forum or one of the topics, just Bookmark that page.)

3) The Web Library Interface (Yeah, that's it, the library as you see it in Galaxy 1.2)

This will cover 2 issues, the old TAG management, that is incompatible/separated from Galaxy 2.0, and the library interface itself.

1. Enter the Forum and click on a random topic or game (now you should know how to obtain this)
2. Click on "My Settings" at the top of the page
3. Click on "My Account" in the "My community profile" section to the left
4. Bookmark This !

That's it, now you can use the old interface (and select it every time you want from the link to the left), clicking on a game will show you the game page exactly as in Galaxy 2.0.

The TAGs used by this interface are the ones of the old 1.2 client, you can use them exactly as in the old client to filter the view and to manage games, but WARNING as them are (still ?) not syncronized with the ones used by Galaxy 2.0, so you should choose if to use them, or the newer ones from the Galaxy 2.0 library interface.

4) The List of installed Games to the left

Yeah, the 1.2 client had a direct link to the left to every installed game, with 2.0 it is possible to obtain exactly the same, the only difference is that you'll have to add every single game to the list by hand.

1. Go to an installed game page
2. Bookmark This !
3. Repeat for every installed game

5) Make the old interface the default one

You can't ! :D

But there are some workarounds:

1. When you select a game page from the old interface, use the Left history button at the top to return to the classic library
2. In Galaxy "settings -> general" select "Starting page = Continue previous session", so at the start of the client there will be a high probability to get the classic library page, if you closed the client with it.
3. You can always click on the bookmark at the left to open the old interface.

I have in mind only two functionalities that are missing on Galaxy 2.0 in comparison to the 1.2, the forum notifications about followed topics and a better TAGs management.

EDIT: added pictures (the forum accepts only files with the extension in lower cases...)

I strongly disagree with this, because... Being able to do this WITHOUT requiring an extra workaround is ABSOLUTELY 100% FOUNDATIONAL BASIC INTERFACE DESIGN. Galaxy 2.0 doesn't even have an OPTION to set this up in any meaningful way. The workaround requires manual addition of every single game in your library one at a time, then constant maintenance of repeated addition of any newly-purchased games. That's way beyond being an unacceptable workaround when it's required to restore the most basic of basic functionality that should be EXPECTED in an app designed for PC users. Which is, in case anyone is forgetting, literally the entire customer base for a PC gaming platform by definition.

This functionality alone is sufficient to say the app isn't in a fit state for launch, and that's before considering several other major problems, some of which are more explicitly service-breaking rather than just impairing core functionality to an objectively unacceptable degree.

Let alone that it's already known how you dislike the new client, I have to point out that you are speaking about OLD functionalities only, without even taking into consideration the newer ones of the 2.0.

You speak about a "manual" addition to the left bar of every purchased game, but the old client put on the left only the "installed" ones, and unless you are constantly installing/uninstalling hundreds of games, it's pretty doable by hand.
Now you can put to the left even uninstalled ones, as a plus, if for some reason you'll need them there.

Also, while I like the possibility to still have links to the left, I find more useful the newer filtered views... with a single click I can see a customized list of games, and this wasn't possible with the old client.

I currently have Galaxy 1.2 running on my system, and can confirm 100% that isn't true. It DEFAULTS to only showing installed games, but you can change that. And even on the default, it's better than the current version offers. Debatable, and more importantly, overruled by the fact that it shouldn't be NECESSARY to do it. Also, I have hundreds of games each on multiple platforms and can realistially expect GOG to end up in that range even if they DON'T fix much, just by being marginally less terrible than most of the competition for another few years.

Also I think it's a bit of a misrepresentation of my position to say I "dislike" Galaxy 2.0 - that implies that my issues are matters of personal prefeerence, when as noted, they're matters of service-breaking flaws and objectively bad design choices. I have valid reason to be discussing serious problems with Galaxy 2.0 but I've openly said - and will continue to say - that I wish the last known stable version of Galaxy (that would be the newest 1.2 build) had a dark mode with a colour palette like the one on Galaxy 2.0 because it looks great. It's just held back by the literal platform-breaking issues and demonstrably missing features in spite of a promise to deliver all features from 1.2 in the new client. They've had 2 years - NOT counting the time they were correctly treating the 2.0 beta as the beta is clearly still is - to fix it, so I think it's a long way past fair to call them on the problems at this stage.

Most probably I didn't remember that option because I could have found no use to have the complete list of games to the left bar when I already had them in the grid view, with the ability to use filters.
And if the Grid view wasn't present because OFFLINE, the not installed games in the list should have been useless because the impossibility to install them, anyway even 2.0 has this function. It is "necessary" to do that only if you really want to use the OLD way to show the list of games, because the 2.0 is perfectly capable to show an automatically updated list of games, and it's also absolutely more powerful with that versus the old client, cause you can have the ability to show different "lists" with different filters.
The only difference is that those lists are not directly in the left bar, but a click away (to make you choose what list to see).

Personally I prefere grids, but it's good to have the choice to mix and match all the kind of views you want, like a "grid view ordered by last time played shooter games", a "list view ordered by name and grouped by tags", etc...

No, it is necessary "only" if you want access to a reliable interface design of a type that is OBJECTIVELY more user-friendly than the options provided without the obnoxiously overdone workaround provided by Galaxy 2.0

The fact that such a workaround necessary in order to regain CORE FUNCTIONALITY is itself a valid proof that the current state of Galaxy 2.0 isn't suitable for it to be the primary user-facing platform yet. Failing to provide a proper user-friendly interface of a type that has been consistently provided by multiple competing platforms AND by your own service previously and continues to be available through other sources is terrible design. Blocking users from setting up such a layout without a significant time investment (which negates the entire purpose of it being MORE USER FRIENDLY than alternative design options is a MAJOR DESIGN FLAW that should have prevented the app from going live. To make things worse, this problem and requests for it to be fixed existed prior to the decision to force the unfinished mess of a beta onto the userbase when it's not ready. This isn't just an opinion, it isn't just me saying "I like this they shouldn't get rid of it", this is OBJECTIVE FACT about design principles. There are literally courses on user interface design which cover it and directly explain why this kind of layout should A-L-W-A-Y-S be made available for users because NOT DOING SO MAKES HE APP OBJECTIVELY WORSE AT THE JOB IT WAS DESIGNED TO DO. And being able to use various "lists" is great. BUT being able to use various lists and sort them freely WHILE MAKING SURE THOSE LISTS ARE ALWAYS VISIBLE NOT GETTING LOCKED BEHIND A "BACK" BUTTON TO REGAIN VISIBILIT ON THEM is a core design principle that's OBJECTIVELY NECESSARY for optimal user access to the exact reason a game launcher exists.

The client is designed to do a lot of things, not related to just launch the games, but can be used easily to launch games directly without the need of a back button...

Example:
Start page = Installed games, so you see the list of your games at the start.

Click play on the game you want to run (you don't need to access the game page to start it)... and when you quit the game you are still in the Installed games page, ready to launch another one, without the use of a back button or changing any page... you opened the client with the installed games list and closed it in the same page without any kind of switching.

You want to see all your games instead of just the installed ones?
Just change the starting page to Owned, and the rest is the same as I explained before.

If instead you use the client also to go to statistics, the store or forum pages, that's another story (still anyway just a click away from your games list), but if you want to use it just as a launcher, the direct interface is there.

Not even a GOOD, but a HALF-DECENT launcher app, not just for games, but ANY launcher app for ANYTHING, needs easy and reliable access to information about the app you are looking at WITHOUT compromising access to launch that app OR any other app. This functionality requires significant unjustified additional actions to set up, and repeated maintenance on purchase of any new software, in Galaxy 2.0, while being provided in an easily-accesible form in 1.2 and older, as well as in most competing launcher apps. It's been a feature in Steam since before GOG existed as a gaming platform. It's a function available in views in several smaller brand-exclusive storefronts. It's somewhat less reliably accessible on Epic, but even that has a more useful and relevant sidebar than Galaxy 2.0 provides, and when you're being unfavourably compared to a platform which took over a year to even implement a cart, that's not a good sign.

Ok, now I'm really lost about your tought... are you still speaking about the left bookmarks to the games ?

Because as I already explained, you can create them manually, if you really want, but are not required because there are faster methods to run things on 2.0...
Just an example, I am now writing in the forum with Galaxy 2.0, but without even changing the page I do
CTRL+F "sacr" ENTER
And "Sacred Gold" is launched.
Or if I really wanted to use the mouse, click on Installed, click on Sacred.
Using a left link don't change too much... click on the link and click on Play... so it's the same as to use the library.

And anyway since you compared Steam and Epic, in my case it works like this:

Loading times = Galaxy FAR superior of both of them (like 10 seconds Galaxy VS 30 seconds Steam and 2 minutes Epic)

Launching a game:
Galaxy = Click
Steam = Double Click (It's mostly the same of Galaxy, with almost the same TAG and search system, but somehow it's less clean than Galaxy and there are too much stuff in the library interface that are occupying space instead of games...)
Epic = Wait the loading of the store page... click library... wait loading of library... click game, and if there are trouble with connection I can't even see my installed games, so I'll have to go manually to the installation folder to launch the exe manually

You seem to be missing the point I've made in literally every comment in this discussion about the fact that having to MANUALLY CREATE EVERY ENTRY to regain access to the most basic of expected features available to some degreee in every competing launcher app is an unacceptable level of effort being forced onto users to get the desired functionality. Of course, this is compared unfavourably to the less than 3 second loading time on the old Galaxy if you revert, but we'll just ignore that. In my experience, with a library about 50% larger than I have on GOG, and the app and all its games installed on HDD not SSD, Epic takes almost no time to switch to library view, and has a sidebar list of the most recent up to 10 games played. Which is still not a proper sidebar list, but is much closer to being a valid option than what Galaxy 2.0 provides. I've also never had Epic fail to connect, and it has an offline mode which you can enable which prevents this from happening.

And since you're so fond of justifying unreasonably-complicated workarounds, you can go to any game's install folder once you've installed it, right-click the game name and click "pin to taskbar" or while the game is running, alt+tab out and right-click the taskbar icon and use the same option, and it'll be permanently accessible without even needing to launch the app. Any Epic games which don't require the app to run, just launch independently with no issues, and any which require the app to enable online functionality will auto-launch it alongside the game.

it's easier to set up a workaround to avoid even needing to open the Epic launcher than it is to set up the workaround that gives you a half-decent sidebar list in Galaxy 2.0, and I consider that to also be an unreasonable expectation to put on users because it, like the workaround required to give Galaxy 2.0 a semi-competent interface, circumvents the entire point of using a game launcher.

None of your proposed solutions actually solve the problems this failure to provide basic functionaliy causes.

It's more clear now what is your point of view, you were used to the left list and you just can't accept to deal with a different way to do the same things.

And while this could be perfectly understandable, because everyone has it's preferences, the wording you use to explain this are wrong, because from what you were telling I was understanding that the client was bad designed and didn't included the basic functionalities to manage the library, but it's clearly not the case, since there are all the intruments to do that and also in a more powerful way. (Excluding TAGs management that is actually incomplete...)

You were used to a motorcycle, then they gave you a car, and you are complaining that it is bad designed because it doesn't have anymore a handlebar... now there is the streering wheel, that not only does the same, but also contains buttons to change the music and answer the phone.

You have all the rights to say "I prefer the old way", just don't confuse a missing personal preference to a missing functionality.

As a side note, on Epic I have absolutely no bar with the last played games, and I can't see any options that speak about it, I always have to go to the library, and anyway when the client have issues I'm perfectly fine in launching the games by hand, i don't want to become stressed because games, them are needed for the opposite purpose.

No, my issue is what I said it is. Which is that there is VERY GOOD reason why the OPTION to have a fully functional sidebar game list WITHOUT needing to manually add every single game you ever buy should NEVER be removed from ANY launcher app on PC. There isn't a valid replacement for that system. The new system doesn't provide the same access and functionality that system does. It is objectively worse at doing what I SPECIFICALLY USE A GAME LAUNCHER TO DO. Which is to launch games. Which 2.0 is OBJECTIVELY worse at by not including a view mode which provides the OBJECTIVELY best functionality for doing so. It's not something which there's room to argue against.

Making up a strawman version of my argument to dispute doesn't negate the fact that UI design courses literally use examples that function the way 2.0 does to show "this is how you do it wrong" when it comes to designing an interface for your app. When I say "it is not just a preference" I actually mean "it is not just a preference" and when I say "objectively" I mean "objectively". I'm not saying that I just dislike it on a personal level when I specifically say with no ambiguity that IT IS NOT JUST A PERSONAL DISLIKE BUT OBJECTIVELY WORSE DESIGN IN 2.0 THAN WE HAD IN 1.2 FOR VALID REASONS I'VE CLEARLY EXPLAINED AND YOU KEEP IGNORING.

The point is that I don't feel what you are feeling, on Galaxy 1.2 I always used the search function and clicked on the result, and the only reason to scroll the left bar was to know what games were installed, just because the store library wasn't able to show that.

Now on Galaxy 2.0, I can see what is installed in a grid view, and to launch things I just use CTRL+F, that works even outside the library, so I just can't imagine how a left bar could be useful to me...

That would be fine IF this was just a matter of feeling.

As noted, that's not the case.

This is an issue of objective facts about how to design a launcher app. And the objective fact is that the Galaxy 1.2 interface provides more effective user-friendly functionality than 2.0 for clear reasons that have nothing to do with personal preferences or feelings.

You "not feeling" the impact of the poor design doesn't negate the objective fact that 2.0's interface is poorly designed. If it didn't affect me personally, I'd still be defending the same position I'm pointing out right now, because the interface would still be objectively worse than the 1.2 interface because of NOT just preferences, but design flaws in the new UI.

Don't mind what I'm putting in this post, I'm using it to test links to see how the client behave to them... seems like the only http links that works on Galaxy are the ones in the store page, while all the others outside of the store will open in an external browser...

https://www.gog.com
http://www.gog.com

https://www.gog.com/forum
http://www.gog.com/forum