Early bird gets eaten by the Nyarlathotep
The more adventurous of you can use those (designed as embeddable) Lua scripts to transform your DRM-free GOG.com downloads into Flatpaks.
The long-term goal would obviously be for this not to be needed, and for online games stores to ship ".flatpak" files, with metadata so we know what things are in GNOME Software, which automatically picks up the right voice/subtitle language, and presents its extra music and documents in the respective GNOME applications.
But in the meanwhile, and for the sake of the games already out there, there's flatpak-games. Note that lua-archive is still fiddly.
Support for a few Humble Bundle formats (some formats already are), grab-all RPMs and Debs, and those old Loki games is also planned.
It's late here, I'll be off to do some testing I think :)
PS: Even though I have enough programs that would fail to create bundles in my personal collection to accept "game donations", I'm still looking for original copies of Loki games. Drop me a message if you can spare one!