Added surface configuration

modules/system/display-managers/default.nix

@@ -7,7 +7,7 @@ let
 in {
   options = {
     settings.display-manager = lib.mkOption {
-      type = lib.types.enum ["sddm" "cosmic-greeter"];  
+      type = lib.types.enum ["sddm" "cosmic-greeter" "gdm"];  
       description = ''
         Specify which display manager to use
       '';
@@ -17,5 +17,6 @@ in {
   imports = [
     ./sddm.nix
     ./cosmic-greeter.nix
+    ./gdm.nix
   ];
 }

modules/system/display-managers/gdm.nix

@@ -0,0 +1,12 @@
+{ lib, config, ... }:
+
+with lib;
+
+let
+  cfg = config.settings.display-manager;
+in {
+  config = mkIf (cfg == "gdm") {
+    services.xserver.enable = true;
+    services.xserver.displayManager.gdm.enable = true;
+  };
+}