Steam Deck

Proton

Use ProtonUp to install custom versions of proton on the deck. You can find this in the Software store on the deck's KDE desktop (open up Discover and search protonup-qt)

Heroic Game Launcher

Heroic is a GUI that wraps both Epic and GOG allowing installation of games from both stores. Install Heroic through discover. Heroic can be added as a non-steam game so that it can be used to launch games from other stores in entertainment mode.

Running Rockstar Launcher on SD

This article explains how to run Rockstar Launcher on Steam Deck