My wife and I are thinking of getting copies, but we don't particularly like using Galaxy, so we'd like to know if you can play multiplayer co-op without using Galaxy?

to play multi you will need a Larian account to join up and track your save progress... Gog Galaxy is assumed in all the help files but not actually needed no

single player multi i.e, single player with a custom party of characters you make yourself does NOT need a Larian account but it helps and of course the game is only currently a early beta build so multiplayer has issues but at least Larian was up front about that {unlike project red and Punk}

Steam Vs Gog is confirmed and Stadia is hoped for, as is Windows Vs Mac

Larian account? Is that new? I never needed one for either Divinity Origin Sin game, but my wife and I played those together (in co-op) through Steam. Maybe that's why.

yes its new for BG3 and yes it is optional [for Gog users at least] but it does two basic things

1. collects crash data about your game | system in the event of a crash which you can choise to send in or just delete... tip = sending it in does nothing because its a known fact Larian doesn't read this data from Gog users

2. allows for their crossplay system and its mostly Stadia that made this needed but Larian also hopes to get Mac vs Windows crossplay going and it will help a lot there as well because Apple and Microsoft doesn't talk to ech other {by design}

a Larian account is free and optional but multiplay = you need it