Most GoG games can be run through steam/proton, but it's a bit tedious. I'm guessing that there are better approaches than what I do, which is to
- Download install files from GoG. For BG3 it's like 27 or so files and one setup executable. Place all of them in the same folder
- Add the install executable from 1. as a "Non steam game". Through the "Games" -> "Add non-steam game to my Library".
- Edit the entry from 2. and add compatibility. (Right click and "properties". "Compatibility" and check the "Force the use of a specific Steam ...". I used Proton 8.0.3
- Run it, and complete the installation. The install allows you to run the game directly, but don't do this. Just exit it after finishing the install.
- Locate the wine installation for the game. Something like
"/home/<$USER>/.local/share/Steam/steamapps/compatdata/pfx/drive_c/GOG Games/Baldurs Gate 3/bin/bg3.exe"
- Edit the entry again from 2., set a nicer name. And the adjusted path above as the TARGET. And full path of the bin-directory it's in, in quotes, in START IN.
You should be able to run it as a normal steam game. I've done this with all gog games I own without much issues. Though, I usually check if it works on protondb.com before I buy it.