I have tried the key combination but I get other windows popping up like the Nav Radio panels?
You are keep pressing the wrong keys, those panels should only come if you keep the Shift key depressed + a number.
The doors are controlled by pressing Shift+E and then RELEASE THE SHIFT, and press 2, 3 or 4 after a very brief pause. This is STANDARD FSX behavior, it doesnì't have anything to do with GSX, and GSX won't change in any way how you open the doors on any given airplane.