Yes, it's in GOG's version as well, it doesn't have all functionalities of Steam's one though.

For future questions I'd recommend game's specific subforum :-)

Thanks.
So, no, not at the moment. Creation tools locked to steam, most if not all mods on workshop only. Disappointing. Maybe it will change one day.
As for game specific forums, mostly they are ignored (even gogs flagship galaxy posts directly to general).

Ignored by whom? These are literally the best resources about game-specific issues out here.