nixos/modules/core/gdm.nix

{
  config,
  lib,
  ...
}:
with lib;
let
  cfg = config.modules.displayManager;
in
{
  config = mkIf (cfg.displayManager == "gdm") {
    services.xserver.displayManager.gdm = {
      enable = true;
      wayland = true;
    };
  };
}