Followup to this:
https://www.gog.com/forum/general_beta_gog_galaxy_2.0/how_to_stop_steam_games_from_showing_up_in_galaxy_20

I'd just like to point out that Galaxy 2.0 is out of early access and this issue still persists, more so the launcher seems to prioritize the Steam versions of games over the GOG ones now which completely boggles my mind. On top of that, I've only yesterday been informed by GOG that I have to manually reinstall the launcher because they were too busy pandering to console players to make sure the f***ing updater worked properly.

The main reason why I've always preferred GOG over other platforms is because it didn't get on my nerves, I could just download an installer, install the game and play it. Even after they replaced the installers with Galaxy it was still fairly headache free so I didn't mind, but since the Galaxy 2.0 beta started it's been trying it's hardest to piss me off. Not just functionality-wise, but the whole principle of it.They've created a launcher that helps people with zero interest in DRM-free gaming be more lazy, meanwhile people like me who actually buy their games primarily from GOG are left to deal with this bulls**t. What is it with game companies nowadays? They love pandering to everyone except their actual audience. I've seen it from other companies too, including GOG's sister company CDP in their marketing for Cyberpunk 2077. Are they seriously this oblivious as to who is keeping their lights on? At this rate I wouldn't be surprised if GOG started selling DRM games soon.

That's not exactly true. Galaxy 2.0 is still beta, but is being forced on users who are opted out of beta testing. But officially it is still in beta, not a full release.

Which means they're actively and intentionally violating user preferences to force it on us.

Doesn't exactly make the situation better, but I think it's worth being clear about what the company is actually telling us, especially when what they're saying is terrible. Being clear about what kind of terrible they're being is important.

Hate this version of Gog. Should not have updated.

Not that they gave you the possibility to refuse: it was "update now" or "update at next launch"
(but you can still reinstall the old one and block the updates)

How to revert back and block the update?

https://www.gog.com/forum/general_beta_gog_galaxy_2.0/how_to_keep_galaxy_v12_client_and_avoid_the_forced_v20_beta_update

1. Uninstall Galaxy 2.0
2. Find Galaxy 1.x installer somewhere (google "GOG Galaxy 1.2.67.58 download")
3. Install that Galaxy and create shortcut on the desktop, but don't launch Galaxy yet.
4. Open shortcut properties and add /runWithoutUpdating after GalaxyClient.exe. See the screenshot below.
(Example: "C:\Program Files\GOG Galaxy\GalaxyClient.exe" /runWithoutUpdating )
5. Uncheck "Launch GOG Galaxy at system startup" in Settings -> General
6. Delete "GalaxyUpdater.exe" file. Just in case, to be sure
(c:\ProgramData\GOG.com\Galaxy\redists\GalaxyUpdater.exe )

UPD: Don't noticed that there is a dedicated thread about this already:
https://www.gog.com/forum/general_beta_gog_galaxy_2.0/how_to_keep_galaxy_v12_client_and_avoid_the_forced_v20_beta_update

@obliviondoll OK, added missing part

Missing from these instructions:
Uncheck the option for Galaxy to launch on system restart. This opening of Galaxy will not use the "/runWithoutUpdating" command in your normal launcher and can sometimes prompt it to update (deleting the updater .exe file should prevent this, but it's worth taking all the appropriate steps you can to limit risk of problems).