Hello!
Since it's on sale right now I'm tempted to get it but I have a friend who owns the Steam version and I wanna play in co-op with him at some point. Is that possible between the two versions?

Yes. The game has Direct Connection and LAN, and should support GOG Galaxy when that launches.

I might be wrong but isn't it a requirement that both players have same version number? So you have to take care this is the case. Versions available on gog a usually a bit behind the version on steam, so you might have to turn off automatic updates on steam to avoid problems.

Yes, that can be an issue. At least until GOG Galaxy launches, whenever Steam updates, there is generally a day or two delay until GOG does. The Steam player can make a copy of the install folder before the update, and play from the copy, if needed (you can no longer disable Steam updates). The next update shouldn't be for awhile, though (as of mid-March, the ETA was "some months").

I know it's been a while, but can anyone confirm if it's possible to play coop between the GOG and Steam versions?