I had this game on GOG with Ragnarok DLC and my friend bought this on steam. We are trying to play together but when we try to join game that either one of us is hosting, we cannot. Game says that incorrect game version even when both of us have latest 1.47.

Any ways where we can play together? Because this situation really suck bad, so bad that because most of my friends are on Steam I will buy my games from there in the future. Gamers should not be punished because Steam and GOG see each others as mortal enemies.

Seems that game/patch versions are out of sync for some reason.

Very likely. There was a hotfix on the 21. on Steam and I assume that does not apply to the GOG version (yet). Sucks 'cause I'm in the same situation as you now.

I'm having the same problem over here. I bought the DLC specially to be able to play in co-op mode with someone who has the Steam version, so I'm a little bit frustrated about the situation

TQ AE & Ragnarök 1.48 - Various Fixes and Improvements and Tech Fixes
released November 23 (found on Steam)

And we play the waiting game.

Can anybody confirm that it is possible at all to use multiplayer between steam and gog?

Everybody who played before the release of Ragnarök can, including me. It's just that Nordic is not delivering to GOG and Steam at the same time.