MedeaFleecestealer: So it's Crate's fault that no one plays multiplayer on GO?. Not sure there's much they can do about that. And Zantai's already said it's up to Steam and GOG to play ball if you want crossplay.
It is implemented in the Galaxy API and a developer has just to use it. Sure its more work for them, but it should work as other titles already show. Other than that it probably isnt easy and takes some fiddling.
Wonder a bit about Zantais posts sometimes, too. He said it was up to GOG to get the latest update on Galaxy, but as far as I recall, they are one of the companies who can upload updates for Galaxy Version directly and soon after I complained, the new version was up. Even proved as we a year ago got a version that solved pet proböems even before Steam got the version. (I nearly only play pet builds this is why I remember)
Other than that I do love Grim Dawn much. Very polished game and devs put alot of love and work into it, but like probably most GOGers, dont really care about multiplayer or play it ;)