I finally got an answer from Free Lives' support since Broforce won't really run on my Linux Mint atm and GOG's support couldn't help me.
https://www.gog.com/forum/broforce/linux_mint_black_screen_on_start
https://www.gog.com/forum/broforce/black_screen_after_developers_logo

So Galaxy is the upload platform for developers now?

...that explains a lot, actually...

God help us.

Half th problem is galaxy, the other half is lazy devs releasing half finished products. Whilst the environment exists to get money before something is down this will only get worse.

tbh this sounds like another of those "Unity engine runs like shit on linux" problems :/
wonder what's the problem with Galaxy here ? I thought there is no Galaxy for Linux yet ??

btw. did you ever managed to fix that fullscreen bug in Star Vikings?

So would GOG have been far better off having never developed a client? Do we need to make sure nobody ever uses Galaxy again so that GOG can thrive?

That was my first guess as well. I may be wrong, but it reads as though Galaxy has a game upload feature for devs and it seems to have problems.
How this could affect the offline installers is beyond me, maybe they are automatically packaged with this as a source?
I don't unterstand it myself and GOG isn't exactly open about its methods. Unfortunately not.
The pre-forever version is the last one that ran fine in full-screen.

Ooooh,it's the end of the world........aaaaaaaaaaaaaaaaaaaaaaaaaah!

https://www.youtube.com/watch?v=Z0GFRcFm-aY

It would've been far better if GOG considered the client a side-project, a hobby, that kind of non-critical project that shouldn't be the entire focus of the company's resources. I'm going by their word that it did bring us a couple of games we wouldn't even see without having Galaxy, so just its existence seems to serve a goal.

However, GOG has made it *the* focus. I don't know what are they up to with Galaxy, a higher userbase of the client? Either way, it seems that's their entire focus now...and they're desperate for some unidentified motive to be achieved.

In either case, GOG would have been better off if they developed the client but not as their focus of using their service even as a developer...I myself don't use the client, or rather I sometimes do but more than often not. I just hope they're not going somewhere down the slippery slope. Or maybe we're already going it.