I did some searching on the fora here and couldn't find a solution.

My problem is that, despite launching Grim Dawn through Galaxy, when attempting to start multiplayer the game claims that I didn't log in to Galaxy nor did I launch it through Galaxy.

I am perplexed. I have tried repairing the game and uninstalling and reinstalling both Galaxy and the game. Nothing had any effect. I am at a complete loss/impasse. Someone please help.

Sounds like a Galaxy problem. Have you contact GOG support about it?

The info I saw on GOG support told me to go to the fora for Grim Dawn itself. So, here I am.

Edited to add: The info I saw on GOG support made it seem as if it were a problem with the game itself and told me to go to the GOG Grim dawn fora (they used the generic "the game").

Well, you could see if anything here helps.

https://forums.crateentertainment.com/t/grim-dawn-common-issues-and-solutions/81943/3

I'm having the same problem, did you ever find a solution?