From a MX standpoint, plugging ground power in with the APU gen on does nothing. The relay from the GP is open, and nothing flows. And if the APU gen is off, then so should everything else be in the plane, so plugging in will only draw a little power for the battery bus and battery utility bus. Unless I am missing something, I don't see it as an issue from an electrical/MX point of view.
And the GPUs don't put out 480V. And if there WERE to be a feeder fault (highly unlikely, but possible), the GCU would open the circuit immediately to prevent damage to the plane.
Feeder fault, under voltage, over voltage, under freq, as well as correct phase rotation are all features of the GCUs as I recall.