Can someone who has a gog version of a game play with someone who has a steam version of the game in online games? There are lots of online games and online co-op games out there that are on steam and gog and I wonder if they can join each other’s lobbies and play each other and etc? Or does one need to have a steam copy to join someone else on their gameplay?
Depends entirely on the game. Some games yes some games no.
But how do you know?
Mostly you have to look it up on a per game basis. Probably just Google the game name and "supports cross play between" whatever platforms you want and someone will know