services.xserver.enable makes X server available (among other things) but the choice of what display server is actually used is up to the display server (GDM in your case). Since you are enabling the GNOME module so you will have gnome and gnome-xorg sessions and GDM will default to the former.
You can change the session in the cog wheel menu on the login screen. Or declaratively preselecting it using services.xserver.displayManager.defaultSession option.