It looks like there have been very specific efforts on this in the past, e.g.
- Submit to GNOME Software as a plugin · Issue #16 · snowfallorg/nix-software-center · GitHub
- RFC: Generating AppStream Metadata · Issue #15932 · NixOS/nixpkgs · GitHub
- Brainstorming Generating AppStream Metadata
Users @vlinkz and @davidak were very active here, apparently. Looks like the greatest challenge is getting AppStream Metadata to flow, then the plugin is almost for free. I also opened a related issue on the GNOME Software issue tracker to bring the discussion closer to the source code.
The Nix Software Center’s Run without installing 
might make for a great feature too, if that is possible to do in GNOME Software, to compensate uniquely for not allowing installation via the GUI. The reason why I stress this so much is that, personally, I have no NixOS config on the machines I manage; they all pull their configuration from a remote repository directly, e.g.
nixos-rebuild --sudo switch --flake gitlab:painless-software/nixos-config
I think this is important, because it makes a device truly disposable (provided your data backup + restore is automatic). No fiddling around with local, unversioned configuration changes. But also, no GUI solution that depends on changing your local /etc/nixos/configuration.nix. 