Avarion: I want to play a game with a friend who has not bought the expansions yet. Unfortunately he gets the message the version of the game is not identical.
Is there a way to temporarily disable the expansions so we can play?
AFAIK, two players with different expansions can play together. What in the game is determined by the host's. So if you have all expansions, while your friend only has the base game, and you host the game, the game might have Halflings/Frostlings/Tigrans but your friend can not start as a Halfling/Frostling/Tigran leader.
"The version of the game is not identical" means that you and him have different patch version. If he is at Steam, he has Patch 1.602 or 1.603 Beta, while you as GOGer have Patch 1.600.
In order to play, you or your friend need to upgrade/downgrade the game to Patch 1.600.
I hope this will help.