Open source software is awesome and in the context of Galaxy 2.0 I really do think that the whole thing should be open sourced. It would allow the community to peer review, conduct security audits and it'd even allow the community to help develop native functionality and bring the client to other platforms such as Linux.

Let the community collaborate with you guys on development and maintaining the client. It's a mutually beneficial relationship and unless there's some proprietary middleware being used... Why the hell not?

Furthermore it would make a nice plus compared to other (very similar) platform clients like steam, epic, uplay, origin, battle net, ...

Linux support has been number one on the wishlist for years, it would be a shame if we still can't use basic GOG features like save sync under Linux with Galaxy 2

I could be wrong. But my understanding of gog 2.0 is to join all the different stores into 1 single client. Not to use gog 2.0 to launch a game through steam client for example. To do that it would require partnership with the respective stores and no way would they let anyone have that kind of access.

No, you were right the second time. Their goal is simply to combine access to your titles under one client, much like Playnite. In that, they are only using other stores' APIs to access library information, which they technically collect but have promised not to sell or share in their privacy policy.

I'm hoping there is at least enough open source integration to add something like GLOSC or other Steam overlay injectors so that every time you launch a game that isn't from your Steam library, you can sorta hack in the overlay and make use of Steam input regardless for full customizable controller support for every game from every launcher.

I really don't see too much value in Galaxy without it being open-source right now. Lutris just seems to be an all-around better choice for now

From my personal experience I would like to disagree, it wasn't a really enjoyable experience for me (but I realize that may be just my special situation/configuration)

Regarding the whole topic:

Disclaimer first: I am no linux enthusiast, but on the private side of things I am a happy (almost) Linux only user and would be a very happy camper if gog would finally release a linux version of Galaxy.

What incentive should gog have to do that ?

To me it looks like the Galaxy client is a considerable investment for gog (both in resources and money) so they need some kind of "revenue" out of it to justify throwing money/resources at it. With gog already beeing not the most profitable business in the world, and the Linux userbase not really playing a significant financial role, releasing it as open source does not look very attractive to me:
Of course people are right when they say, that making it open source would possible lead to people adding functionality and squising bugs for free. But this is not a given thing, this may very well end in people getting "all in" at first and then interest in the community slwoly dwindeling away again. Especially as competition is out there already, both for Windows and Linux - which may cause some potential contributors to ask the question why they should invest time in somehting "company owned" when there are other, scucessfull "community owned" projects.

Gog current strategy looks more like to go in the direction of "if you can't defeat your enemy, try to embrace him" > the sneaky route to keep gog in the back of their potential customers heads, hoping they lean more towards gog in the process of buying a game.
But with the code out in the wild, there is also the possibility that people start creating an "unbranded" version of the client without any mentioning of gog in the UI in the first place.

So where is the benefit for gog in doing so in their current situation (this is not meant rhetorical, I really want to know) ?

Money. This is not a side job a few IT enthusiasts are throwing up together in there free time. This is a project funded by a commercial company.

If you're using Linux, then Galaxy probably isn't a good choice for you, unfortunately. They've shown almost ZERO commitment to anything Linux, be it getting Linux versions of games to their store or supporting a Linux version of their client. For a DRM-Free Store that is an odd choice, in my humble opinion.

Why ? Don't see a necessary connection between DRM-Free and Linux.
In the end its all about money: supporting Linux needs more expertise and resources with only a niche group in a niche market benefitting from it.

I find it more obscure that a lot of people that prefer Linux have no problem supporting a DRM-laden store like Steam.

Steam supports them. Why wouldn't they decide Steam is the way to go? It's not the preferred method, but in the end, the options are fairly limited.

A free and open platform fits into the theme of DRM free far more than the spymaster general's preferred operating system, Windows. Or anything from Apple, another company who loves to profit off their users' data. Hell, at this point, the only thing less shocking would be full Android support (while ignoring the Linux counterpart since Android is based on Linux).

Valve has a lot more money to spend on people doing the support and development work. And with Gabe Newells fear of Windows getting a closed system like iOS there is more "operational motivation" on Valve side to invest money into the Linux support even if there is no financial benefit to be gained.
Gog barely gets along with their business model, supporting Linux just does not fit really well budgetwise in their business strategie.
I have no experience in running a company, but if I had to choose either to spend money on supporting a Linux client (and this does not mean only spending money on people coding the client) and gain neither a significant rise in sales nor in company reputation. Or maybe lower my cut in deal with a publisher/developer which game I think will draw people or at least sales in. I know what I would choose, despite wanting a Linux client since the old Galaxy client was announced.

That's the thing though. Even if they just open sourced it and told the Linux community to support it themselves, they totally would and GOG would benefit from additional sales. Client could get some things like big picture mode too out of the open sourcing. Seems like a win/win all around to me.

But why ? There is no automatic process that leads to the scenario you are describing, and there is also no guarantee for those additional sales, as a lot of Linux users seem to not care about DRM as long trhey can play current (AAA) titles.
Maybe you could give some reasons why you think this would happen ?

Also, considering what a small minority Linux users already are, the income would not realy be felt.

Heck, the only reason Steam has any Linux support is the truck loads of money they get fro mall the card/hat/lootbox trades. They can afford to throw money at the wind so a few Linux users could feel happy.

GOG? Not so much.