The GNOME section of the Nixpkgs manual explains what you need, in this case it is adding appropriate paths to XDG_DATA_DIRS environment variable. wrapGAppsHook4 does do this correctly but only for built derivations, not devShells, so it will not help you with cargo run.
There is no mechanism for shells at the moment, you will need to apply a hack like shellHook from Python + Qt woes - #2 by jtojnar with gappsWrapperArgs instead of qtWrapperArgs if you want to make it work.
Do not do that, it will break GTK programs if the schema is ever updated.