i think the standard boeing has a APU build in...
That's exactly what I've said: FSX has introduced an APU system, but nobody except the default 737 use it, and nobody will want a GSX feature that works only with 1 default airplane.
And, it would be very tricky supporting each 3rd party airplane, since each one have its own way to simulate the APU, it's way more complex than reading custom doors with XML variables, because there are surely tied to an XML variable (so we can put an XML expression to test it), but the APU doesn't need to, so it's likely an in-memory variable that should be accessed by reverse-engineering the gauges, which would stop working as soon as the developer releases an upgrade. We could basically do it only for the very few airplanes that have an SDK, like the PMDG which, incidentally, already has its own visible APU vehicle, so it wouldn't make sense to add a new one in GSX.