So I just started playing this with some of my friends, but one of them hasn't bought the game. We all have it on Steam, so my question is, if he buys it from GOG, will we be able to play together?

Any pointers are welcome, so thanks in advance!

EDIT: Post removed because of false information.

From the store page:
"Please note: The GOG Galaxy Client is required to access online features.
Cross-platform multiplayer is supported."

So have you tried the multiplayer aspect of the game? Does the cross play actually work?

No, sorry, turns out my friend's computer can't run the game, so we weren't able to give it a shot in the end. :(

That's good to know. Makes the game instantly more interesting imo.