It's not impossible to run gog games through the proton compatibility layer of steam. In short:
1. Download gog install files into the same directory
2. Add the install executable as a non-steam game, and enable proton compatibility before starting it up.
3. Run the installer and complete the installation. Gog installers typically give the option to run the game from the installer, but that will only work once.
4. Find the path of the installed game, and the working directory of that executable by searching in ~/.steam
5. Edit the entry created from 2. with this exec path and working dir, and look up possible args from protondb.com
1. Download gog install files into the same directory
2. Add the install executable as a non-steam game, and enable proton compatibility before starting it up.
3. Run the installer and complete the installation. Gog installers typically give the option to run the game from the installer, but that will only work once.
4. Find the path of the installed game, and the working directory of that executable by searching in ~/.steam
5. Edit the entry created from 2. with this exec path and working dir, and look up possible args from protondb.com