There is now a default GPU on many airports in X-Plane that you can request via the ground services menu.
I realize that this GPU just drives up, stays for a while then leaves again (and that it´s presence does not coincide with the availability of ground power), but we did not want that GPU to "clash" with the default one.
Also, often ground power comes not from a cart (this is nowadays the exception), but from a fixed wire at the jetbridge or from a ground socket.