anchor.diff:  
       programs.dconf.profiles.gdm = "${gdm}/share/dconf/profile/gdm";
   
  +    environment.etc."gdm/custom.conf".text = ''
  +      [daemon]
  +      ${optionalString cfg.gdm.autoLogin.enable ''
  +      TimedLoginEnable=true
  +      TimedLogin=${cfg.gdm.autoLogin.user}
  +      TimedLoginDelay=${toString cfg.gdm.autoLogin.delay}
  +      ''}
  +
  +      [security]
  +
  +      [xdmcp]
  +
  +      [greeter]
  +
  +      [chooser]
  +
  +      [debug]
  +    '';
  +
       # GDM LFS PAM modules, adapted somehow to NixOS
       security.pam.services = {
         gdm-launch-environment.text = ''
  
positive.code: 
      services.xserver.displayManager.gdm = {
  
        enable = mkOption {
          type = types.bool;
          default = false;
          example = true;
          description = ''
            Whether to enable GDM as the display manager.
            <emphasis>GDM is very experimental and may render system unusable.</emphasis>
          '';
        };
  
      };
  
negative.code:         session  optional       pam_permit.so
        '';
  
        gdm.text = ''
          auth     requisite      pam_nologin.so
          auth     required       pam_env.so
  
  