Whatever works on Arch should work on SteamOS as well.There are at least two launchers on Linux that support GOG:
Minigalaxy (specialized on GOG)
and Lutris (support for many platforms)A simple GOG client for Linux that lets you download and play your GOG Linux gamessharkwouter.github.io
Play all your games on Linux. Lutris is an Open Source gaming platform for Linux. It installs and launches games so you can start playing without the hassle of setting up your game. Get your games from GOG, Steam, Battle.net, Origin, Uplay and many other sources running on any Linux powered...lutris.net
Let's see how the support on the Steam Deck will be for such solutions.