I've seen conflicting information around the net about whether this game is single or multiplayer - it seems there may even be different versions of the game, some multi, some not.
Is the GOG ver single or multi (hopefully coop)?

Commandos: Behind Enemy Lines and Beyond the Call of Duty are single-player, real-time, isometric, tactical, World War II, stealth games.

Actually Korell, all of the Commandos games have had multiplayer. It has a co-op mode, where each player selects which commandos they wish to control. I played both the original and Beyond the Call of Duty many times over LAN.
Can't speak for the GOG versions, but I assume they will be the same as the retail game I bought years ago.

Yes, there's multiplayer.

I never got around to having a go at the multiplayer part.
Any good? Worth a go like?

i played it coop years ago with a friend, it was very cool.
back then there was some problems with games becoming out of sync, i don't know if that still exists. i remember there was a problem with the quick load/save when you start a new coop game (after it crashed or sth) and we had to manually copy the quicksave file around to resume a mission, so we didnt have to restart from the start of the mission.
a non-technical "problem" is, that there are many parts in the game which can/must be solo-ed by the Green Beret or the Spy, so it might become a bit boring for other players. but afair you can change which player controls which commando during a session so you can take turns during those parts.
tl;dr version: besides some minor problems, the coop is highly enjoyable