add cosmic modules, probably won't switch to it

modules/boot/greeter/gdm.nix

@@ -0,0 +1,23 @@
+{
+  inputs,
+  outputs,
+  lib,
+  config,
+  pkgs,
+  ...
+}: let
+  inherit (lib) mkEnableOption mkIf mkOption types;
+  cfg = config.horseman.boot.greeter.gdm;
+in {
+  options = {
+    horseman.boot.greeter.gdm = {
+      enable = mkEnableOption "GDM Greeter";
+    };
+  };
+
+  config = mkIf cfg.enable {
+    services.xserver.displayManager.gdm = {
+      enable = true;
+    };
+  };
+}