I would like to have DRMfree verion of this game.
Im just not sure how coop works in this version.

You can simply invite anyone from your friendlist into your game and play online together in Steam version. Is this feature also for GOG Galaxy's friendlist?

This feature is not available on GOG Galaxy. However, you can achieve the same results with Parsec. This software does the same thing as Steam, except you can use it with every game, not only Steam ones.

BUT, I really do not recommend you buying Binding of Isaac on GOG. It lacks a lot of things: Achievements, Proper mods support, Workshop, multiple bug fixes and Daily Runs (so theoretically you cannot 100% the GOG version of the game).
The only reason to buy it on GOG is if you want a DRM-Free copy of the game.