Can Galaxy 2.0 show you games in your library from other stores when browsing the GoG store or does it only show games from GoG?

For example, I have 400+ games on Steam (Thanks Humble Bundle). There's no way I can remember all of them.
The last thing I want to do (and I'll admit, I have done this before) is browse the GoG Store, see a good deal on a game, purchase it only to realize I already own it on another platform.

If it currently doesn't do this, its something that would be a very useful feature to implement in the future.
I understand it might cost GoG sales because of it, but being more consumer friendly would go a long way.

It is literally written in the very home page of the website dedicated to it
https://www.gogalaxy.com/

You can import other games into GoG Galaxy. The games still require that you have the client of that store installed and GoG doesn't update them for you but you can execute the "download" and "play" actions from within Galaxy (just tested with my steam account).

Galaxy comes already with some of the popular stores integration loaded but you can also add other less popular store by using community plugins, here is the full list: https://github.com/gogcom/galaxy-integrations-python-api/blob/master/PLATFORM_IDs.md

Just to clarify, when you launch a game belonging to another launcher, you're launching that copy by using that launcher. GoG Galaxy simply act as an aggregator so you have the whole library of your games in one place, which is definitely a great feature anyway.

GoG is not giving you access to their copy of a game you own on another e-shop for obvious reasons, one of which being that the GoG version would be different version (drm-free)

You seem to have missed the point of my question.
I understand all of what you have said. (b.t.w, Galaxy is more than just an aggregator for your library. Its also the store front for GoG).

What I'm saying is, if I link my accounts to Galaxy it knows what I own from the other stores. Its right there in my library (or collection), however, if I browse the GoG store from within Galaxy 2.0 only the games I have purchased from GoG have a 'In Library' tag on them.

What I would like to see is another tag letting me know that I already own the game on another platform (Or at least an 'in collection' tag).
Galaxy should be able to get this information from either the linked stores APIs or even checking the local collection.

There is an 'All Games' list after all so the data is there.

I get it now. What you are asking is a two-way communication between the "store" section and the "library" section.

It would be nice but if you indulge me for a moment I'll dress up as a marketing rep from any generic company and tell you that:

1. It would hurt the sales of GoG
2. Technically speaking, the store page shows you what you already own of GoG products. That is its scope and it is doing it properly.
3. The GoG Store is not selling the same version of games you have anywhere else because they are always DRM-free version

You can always search your Galaxy 2.0 collection yourself before making a purchase.

It might, but its better for the consumer and in turn the 'good will' will be better for GoG in the long run.
Purchasing a duplicate copy of a game is generally 'wasted money'. There is very little point to owning a game twice. (There will be reasons why someone might want duplicate copies but I would bet its not the norm).

GoG might miss out on the sale of that particular game but it also means the consumer still has that money available for something else which could potentially mean that money is spent on another game (non duplicate) at GoG. I understand that, and I'm guessing its just pulling the data from the website and isn't integrated into Galaxy very much at all, however the possibility is still there to have the feature added. I would suspect that when it comes to duplicate copies of a game DRM isn't much of a deciding factor. The consumer already owns the game on another platform.
It might have been by choice because they don't care about the DRM, it might be because it wasn't available on GoG at the time, or maybe its an old game that was purchased before GoG was around.
Regardless, the consumer should be able to make an informed decision as to whether they want to repurchase the game from GoG.

Programs like Galaxy 2.0 (where your games are aggregated into a single launcher) aren't new.
You have software like Launchbox and Playnite... Hell, AMD even tried it ages ago with their AMD Gaming Evolved software (re-branded Raptr).

The reason programs like this came into existence was because of the fragmentation of the gaming market.
You have so many launchers and market places now its hard to keep track of what you own and what you don't.

So on one hand, by aggregating your games into a single launcher GaG Galaxy 2.0 is trying to combat the problem, and on the other hand, they are contributing to the fragmentation issue by not showing games owned on another platform... Which to me seems counter intuitive in a piece of software designed to aggregate your games.

Its not something I would expect when browsing the store through the website, but there's no excuse for not having it within the Galaxy 2.0 software (other that trying to take advantage of the consumer purchasing duplicates).

I would love to see the Twitch library added to GoG Galaxy 2.0.