Gears 5 from The Coalition and Xbox Game Studios recently had an update that should make it work better on Steam Deck with Proton. Really nice to see another Xbox published title look to improve things!
The update notes were short and sweet noting "Updates to provide access to Steam Deck" and no other information. It was also recently rated as Steam Deck Playable by Valve using Proton 8. Without any issues now, you should be able to just load up into the campaign.
However, the multiplayer won't work out of the box but simply switching to Proton 7 should fix that. Seems like this may be a regression in Proton 8. So for now, for the best experience, just go into the compatibility settings and set Proton 7. There is a bug report with various info noted already confirming Proton 7 works so hopefully Valve will be looking into it.
It's not unusual to see newer versions of Proton have issues, which Valve eventually fix up. We've seen a few Proton Experimental releases recently (like from June 24th) that continue fixing up regressions. This happens because when Proton jumps a major version like 7 - 8, there's just thousands of changes in the Wine version it's based on and it would be impossible to find all issues. So no doubt sometime soon it will be fixed up for the latest Proton too.
You can buy it on Steam.
See more from me