If it's any consolation, Valve also makes it pretty easy to play GoG games on Deck. You can run GoG installers in Proton and set them up with their own virtualized C:/ drive like other Steam games. They do a good job ensuring that even if a competing store came out, users would have proper access to it. That's commitment to customer satisfaction worth supporting, but don't be afraid to support devs outside Steam either!
Kinda? You then have to figure out the correct graphic settings.
I have BG3 on GoG (backed on kickstarter way back) and unfortunately it runs terribly on the Deck, to the point I'm not playing it right now
tbh, Baldurs Gate 3 runs pretty terribly on my Ryzen desktop also.
The issue is that GoG does not support Linux.
This is a GoG problem which could be solved, but they are uninterested in my money.