You can go to the desktop mode and install the GOG launcher. Then after installing your games, open Steam and add the games as non-Steam games. They’ll show up in game mode as well.
The Heroic Launcher can add your games from GOG, EGS, and some others iirc to Steam automatically. I believe there are other, supposedly better options too, but I can’t remember the name(s) right now. Also don’t remember what (if anything) was wrong with Heroic.
Okay Ryan George