

If you don’t want to go the Heroic route (you really should go the Heroic route) you can
- download the offline installer from GoG
- add the installer as a none-Steam-game to Steam
- select Proton for compatibility
- run the installer
- add the installed game’s exe to Steam as a none-Steam-game
- select Proton for compatibility
Trouble with that method is that Steam always creates a new Wine prefix (a kind of fake Windows drive) every time you run a new none-Steam-game with Proton. They can pile up and take unnecessary space away. It also makes it harder to install dependencies or mods or add ons.
So yeah, Heroic is the way.












That’s because every time you run a new program with Proton on Steam it creates a new Wine-prefix (fake Windows drive). So when you run the installed battlenet.exe it creates a new Windows environment where Battle.net is not installed.
But the installer is using the environment where Battle.net is installed and apparently it has a function to run Battle.net when it detects that it’s already installed.