You are not doing anything wrong: GSX GPU is just cosmetic, because there's no way to control the availability of external power programmatically.
This won't be a problem because in MSFS 2020/2024, there's a "magic" external power always available on every gate but, that works only with airplanes using a standard electrical system. Those using their own internal simulation usually either have their own GPU OR, if they have some GSX integration, they link the availability of external power to the GSX GPU connection status.
