Nobody?

Still the same issue, even after 4 years have passed. So much for the "fully optional GOG Galaxy" as GOG puts it. If it was fully optional, internet multiplayer would just work.

While I can understand that some kind of server browser is necessary, the game could at least give the player the option to join a game via IP address.

I'd even install GOG Galaxy, but it simply doesn't work with Wine and CD Projekt doesn't care about releasing a native Linux version of it, even if the software is probably only using cross-platform programming frameworks.

This is something you should take up with GOG since Crate have said they don't support Linux for the game. Suggest you head over to the General Discussion forum and look for the Galaxy 2.0 thread to suggest Linux support there.

I actually got this to work smoothly after a nightmare session.

I'll post a short guide soon.

Buy it from Steam. Plays fine in Linux Steam client using the "Steam Play" Proton version of Wine. (The base game, at least. Haven't tried expansions.)

I did what I always did ever since I did my complete migration since last year: installed Galaxy in a Wine bottle and then installed the games in there, using Galaxy to launch the games as always. Either install them through Galaxy directly or run the normal offline installer inside the Galaxy bottle while it is running.
All my Windows games run like this in the same bottle with automatic updates and cloud saves through the same Galaxy instance.

The only caveat I have found would be disabling esync. The Lutris Galaxy installer script enables that by default and it's really not a good idea for a lot of games.

I never really tried multiplayer before but I could just host a session. The alternative would be a GOG version with multiplayer pruned out, I would guess. I wouldn't really care, since I'm here exclusively for singleplayer experience. Too many developers had really bad experiences with multiplayer in the past.