Proton GE is a custom build of GloriousEggroll's custom distribution of Valve'scompatibility layer for running Windows games on Linux through Steam.
This repository contains recipies for building Proton from source on top of freedesktop-sdk in flatpak format, intended for running from the Steam flatpak.
This unofficial build isn't supported by GloriousEggroll nor Valve and wasn't tested with all possible games and cases. It can behave differently from upstream builds. Use at your own risk.
First add Flathub repository and install Steam from there, if not already. Then run
flatpak install com.valvesoftware.Steam.CompatibilityTool.Proton-GE
Launch Steam flatpak and select "Proton GE (flatpak)" compatibility tool from drop-down list.