This game is both available on Steam and GOG, and I was wondering if it's possible to play with Steam users?

I have no idea to be honest. I own the game on Steam and I'm seeing users who I can't add on Steam, maybe they are on GOG or maybe I don't know what I'm talking about. I'm online now as 'jeffkiwi5' if anyone wants to test it.

Yes! Absolutely. The Steam and GoG version are cross-compatible. However, Steam has easier access to mods and maps via the Workshop. GoG users will have to find and install maps and mods via ModDB, under which circumstances you have to hope that the modder has uploaded his content elsewhere.

I apologize for the much delayed reply. I'm much more active on Steam, as are most of the modders and veteran players. I'll try to be more active on these boards for help and support purposes in the future.

If this is on the GOG side, that'd be because there aren't any multiplayer maps out for that version yet so far as I'm aware. If you're trying to port maps from the Steam copy of the game I would guess that you'd need to preserve the name of the folder the map is in and paste it to the /mods directory, but I have yet to try that myself.