Your question doesn't have anything to do with GSX, which doesn't control the airplane doors in any way so, it's really how question about how to use the PMDG 777.
What is probably happening, is that you don't have power (electrical and/or hydraulic) to open the doors, because you turned off the engines without enabling an alternate source of power, such as the APU or the External power, so you cannot open doors without such power.
But of course, this doesn't have anything to do with GSX, it would have happened just the same, if you turned off engines without boarding. It's just that GSX requires you to turn off engines in order to start boarding, so maybe you linked the two things being consequentials, but they aren't.