In theory you can run all your GamesPass games on a Steam Machine in the same way you can run arbitrary games through Proton, which is what Steam is doing.
What a wild world it would be, if Microsoft release a GamesPass client for linux so it can try and get a slice of all this new linux gaming happening on SteamOS.
In theory you can, in practice Game Pass games are distributed in such a way only Windows can run them. You can use Game Pass Streaming which is fine when at home, and entirely useless when on a train using a Steam Deck.
The ideal would be MS just selling Game Pass subscriptions via Steam but I expect we'll see that happen shortly after hell freezes over.
What a wild world it would be, if Microsoft release a GamesPass client for linux so it can try and get a slice of all this new linux gaming happening on SteamOS.