I'm trying to set myself up to practice setting up from the A/C at the gate unpowered all the way to takeoff, but I can't seem to get ground power available at most of the airports/gates. Is this something that can be activated so that ground power is always available regardless of location?

What aircraft add-on are you talking about?

It's the PMDG 737-800 addon.

ground power should always be available as long as the parking brake is set. not sure if the engines have to be off or not, but just turn the ground power on via the switch on the overhead panel