Achievements were already confirmed and supposedly will work retroactively. Most of the games you can look up their lists on Xbox related websites and some have in-game trackers like Red Dead Redemption 2. Only Gylt and Kine have unknown achievement requirements.
GOG library integration would be nice. I know people have been able to do unofficial integrations like battle.net so maybe a community member could figure that out by using game links or something with a little help from GOG.
Yeah, I'm sure the Google team could dip their finger in a lot of pies and get something worked out, even if it's something to work towards in the future.
I would love achievements too, as I am an achievement hunter haha!
There can't be enough of them!
Really hope it'll be implemented soon, at least I have left feedback for this in the Stadia App, so let's see!
FYI: Achievements feature is now available within Stadia and you can now see your progress.