wrote on Jan 12^th, 2013 at 1:36pm:

Remeber that the aircraft is still perfectly flyable even without the nice addon from FSXpaul. If you loaded a flight with the engines running, then cut them and turned APU on you can stay at the gate for hours, atleast until you run out of fuel. ;)

With all the tesing I've done with my new gauge, I've never had the APU circuit work correctly doing what you've described.
Maybe I'll try some more  :-/

[edit]I disabled my APU/Grd Pwr gauge, loaded up with a running aircraft, shut down the engines, started the APU: didn't work. Same with the ground power.
I have FSUICP on my system and had the battery set to 'indefinitely' so I never noticed the APU/Grd Pwr issue before.[/edit]