PC Game Pass and Microsoft Store games now have an unofficial way onto the Steam Deck that isn't cloud streaming. Xodus Nexus is an open-source Linux launcher that downloads Microsoft Store games and runs them locally through a modified Wine/Proton runtime, with no Windows install needed. You still need a Microsoft account and a valid purchase or Game Pass subscription.
It's built on the existing Xodus project, which handles Microsoft sign-in, licenses and downloads, while Nexus adds the interface for browsing, installing and launching games.
Version 0.3.6 is the Steam Deck update. It adds experimental controller navigation and lets you add the launcher or individual games to Steam as Non-Steam shortcuts with artwork, so they show up in Gaming Mode. Some sign-in windows and system dialogs still need touch or a mouse. Version 0.3.7 followed today with diagnostic logs for sign-in and install problems.
The developer lists Gears of War: E-Day, Forza Horizon 6, Palworld, Deep Rock Galactic, No Man's Sky and DOOM Eternal among the games working on Linux. That's the developer's claim, and testing was done on virtualized SteamOS because they don't own a Steam Deck, so it doesn't promise the same results on Deck hardware.
There are limits. Older UWP-based games aren't supported, the Steam shortcuts need the native Linux version of Steam rather than the Flatpak, and some titles have sign-in problems, missing online features or crashes. Some early users report working launches, while others have hit installation and account verification issues. The project is independent of Microsoft and Valve.
My take: running Game Pass PC games locally in Gaming Mode would be a big deal for the Deck. With testing done on a virtual machine and not a real Deck, I'd treat it as experimental for now and expect some games not to cooperate.