If I had a game say on GOG instead of Steam, how seamless or usable would that be please?

You can install GOG games the same way you do on any Linux desktop from the Deck's handheld mode (usually through Lutris or Heroic). After that, you add the games to Steam as a "non-Steam game" and it'll show up in handheld mode just like any Steam game.

Installing new GOG games from the GOG library isn't quite as easy as installing Steam games is, and you don't get the extra testing and verification for running games on Linux that you get on the Deck.

You do get all of the benefits from the Steam UI, though, like controller remapping and game specific performance profiles.

Not all games from other platforms work out of the box, even if the steam version does. Although my main issue there is with epic that doesn’t provide Linux versions even for games that have them. But then again the launcher is also a windows executable.

Mildly annoying to setup sometimes, use Lutris or Heroic as others said but once it's done it's more or less the same as the Steam version. Lutris and Heroic let you easily setup shortcuts to Steam so it will appear in the game UI, you can use all the same features as you would for a Steam game, framerate limit etc.,. Occasionally you can run into issues with the GOG version that don't crop up with the Steam version but usually the Lutris installer does a good job. Heroic is a bit hit or miss IME. Epic and EA games work too but sometimes DRM adds an extra layer. It's about as complicated as getting a game from say 2005 working on a modern PC.

Seconding Lutris. Just install Lutris from the Flatpak store and login to your GOG account.[0] I have played a couple of games this one. The only one that wouldn't install was the first Jazz Jackrabbit which I didn't debug. I have a feeling it was because by default it used the Linux installer rather than just running the Windows executable via WINE/proton.

[0] https://github.com/lutris/lutris/wiki

Like others mentioned, it’s totally possible to use another launcher. I have Cyberpunk on GOG for example and a couple games installed I bought on Epic.

But, the Deck absolutely 100% works the way Valve intended: I find myself preferring to purchase games on Steam because it’s easy, even if it’s a couple bucks more or I have to wait for the Steam sale. It’s just a better experience.

My Steam account is possibly most oldest active account anyways and I have always preferred it, but if you primarily use another store it might be a bit annoying to have to do all the manual steps for the majority of your games

> My Steam account is possibly most oldest active account anyways and I have always preferred it,

You just made me feel old. I think my Steam account is even older than my Gmail account. It has accompanied me through several phases of my life... I first created it using a very silly username, later I grew up and became extremely ashamed of it. I remember frequently checking if it was already possible to change the username, but they never allowed it. And now that I'm older I just laugh about it as a memory of my teen years.

Installing them can be annoying (switch to desktop mode, run Heroic Launcher, find the right Proton version that works), but after initial setup they run seamlessly. Heroic Launcher also now supports GoG Cloud Savegame sync so it hasn't been seriously problematic.

(About half the games I play on Deck are from GoG.)

You can just add the launcher as a non-steam game to make it a bit easier.