Basically every time I go in, it lists ALL games (from Steam and other sites). And every time I have to go in to filters and select GoG it only shows those.

Is there a way to set it up to do that by default? When I use Galaxy, I ONLY want to GoG titles (I load my Steam stuff through Steam).

I'm pretty sure that's how it works by default and you changed it. It wouldn't even know your Steam login unless you gave it.

I mean, I synced all my accounts and such. So the only way is to unsync everything?
Because there really should be a way to only show GoG games.

Click the Bookmark icon to save your filtered view. The bookmark gets added to the sidebar on the left.
You can also click the cogwheel - Settings - General, and select Starting Page: Continue previous session. If you close the application on the filtered view, it will open on that same page next time.

GOGs bookmark solution is actually one of the better ones. Works like a charm, also games can be added, which I usually do for the game I am currently trying to complete.