This isn’t a GNOME-related issue. You’re learning the basics of writing Nix configurations here.
In Nix,
a.b.c = value;
means exactly the same thing as
a = {
b = {
c = value;
};
};
Get used to translating between the two mentally. When you see this The option `whatever'` does not exist. error, that means you’ve written something that translates to a b.c.whatever path that is not what it should be. You might be missing an outer level, and the correct option is a.b.c.whatever. You might be missing an inner level, and the correct option is b.c.d.whatever. Find the correct path for the option you’re trying to set using search, and then patiently examine the line in your config file that is producing the error message to see how the option you’re setting needs to be changed to match the correct path.