GSX doesn't change in any way how you open a door on any given airplane. The only thing GSX does, is to *check* if they are open or not, but it never try to control them, and installing GSX won't change the slightest bit how you operate doors on airplanes.
With default airplanes, the default command to open doors other than the first one, has always been Shift+E and then (after a brief pause) a number which can be 2, 3 or 4, which would open the cargo doors, the service doors and the 4th is not used on default airplanes.
With 3rd party airplanes, they might either use the standard commands, or have their own custom controls.