With default airplanes, provided their profile is correct, GSX will automatically close/open doors, even if you normally can't from within the sim.
With 3rd party airplanes, that's also depending if the airplane is supported, and in some cases, like PMDG, requires custom coding with GSX, which does work. However, with 3rd party airplanes that have custom code for doors, there's always some way to open doors.