DaCostaBR: Não sei parça, nunca usei esse recurso, mas se for pra perguntar em português sugiro que use o fórum br que tem mais chance de achar gente que entenda:
https://www.gog.com/forum/general_br He's asking how to play with a friend, didn't specify, possibly a Galaxy thing. I never used that feature, mostly stick with singleplayer or couch coop, but if anyone else has an answer I could translate for him.
then maybe the game's name could have helped...
as some (very rare, too rare) GOG games do have online multiplayer features that work plain TCP-IP style, and even cross platform as well (ex i know about: grimdawn, windward...)
some has local multiplayer as in "local network" multiplayer, which can easily be turned into online multiplayer using vpn feature between trusted peers/friends
some have local multiplayer but all on same single machine and now it becomes harder to do anything remote