If so, how is it working with your GOG library, especially with pre-2010 Windows games?
Well you'll need to rely on Heroic launcher if you want any shred of convenience. Achievements won't work obviously.
Otherwise you'll have to manually load the setup onto the device, add the setup as a steam game, assign a compatibility layer, "play" the setup, install the game to some fake Windows Directory, then go back to your setup "game" and change the link from the setup to the game's executable, by navigating weird Linux directories, random IDs that Steam assigns (have fun figuring that one out every single time you install a game, especially since you navigate via stupid Steam UI and not Dolphin, thus you cannot sort by last modified date and have to look it up separately), then fake Windows directories till you finally reach that executable.
Of course if you're already a Linux user, you may be used to dealing with all this madness.
I personally dislike Steam OS, the inability to gain Achievements, and lack of a keyboard. We need to tinker so much and there's no keyboard! The on-screen keyboard doesn't even have a CTRL key! I got fed up and just went back to GPD, who have been in this game far longer than Valve. I have recently ordered a GPD Win 4. It should arrive in a month or so. Steam Deck's only advantage is the price IMO.
If I could only afford a Steam Deck, I would have installed Windows on it by now, rather than sacrifice achievements and deal with that madness. I mean sure, the Heroic launcher simplifies things a bit, but you have to trust a 3rd party client with your account.