We have a lot of power-hungry kit and a small APU so we can only run some of the kit but not all of it, off the APU. Or we can run the A/C - badly off the APU. It starts the engine and runs the flight controls okay, but that's not what we're after.
Current preference is to run the kit off a GPU, and the A/C off the APU but it doesn't really give good enough cooling effect. Most places we go we can't get a GCU - and when we can they're usually heavily in demand, or unreliable, or both. So a compact air-portable external GCU that we'll only use externally and on the ground (so no particularly certification issues) combined with the APU running the aircraft a/c is what we'd like to do. GPUs to run the kit on board is at-least quite easy to rent wherever we are.
But first catch your compact GCU. I'm just hoping that somebody, somewhere, makes such a device and I've just not found it yet.