nix/modules/wm/gdm/default.nix

{username, ...}: {
  services.xserver.enable = true;
  services.xserver.displayManager.gdm.enable = true;

  services.displayManager.autoLogin.enable = true;
  services.displayManager.autoLogin.user = username;

  systemd.services."getty@tty1".enable = false;
  systemd.services."autovt@tty1".enable = false;
}