Set /etc/shells so GDM will be happy

modules/core/user.nix

@@ -41,5 +41,8 @@ in
     ignoreShellProgramCheck = true;
   };
 
+  # Set /etc/shells so GDM will show our user when ZSH is selected
+  environment.shells = with pkgs; [ pkgs.${shellCfg.defaultShell} ];
+
   nix.settings.allowed-users = [ "${username}" ];
 }