Can we play from Gog with Steam users and viceversa? Or is the game userbase limited to the users of the pĺatform in which you bought it?

Ty.

Steam players can play on non-steam servers with the launch option -nosteam
Mods will have to be installed manually since the non-steam version obviously doesn't support steam workshop

GOG players cannot play on steam servers though, because it requires steam authentication for security, friends, workshop, etc...

So the answer is, it depends on the server.