Please note that your question doesn't have anything to do with GSX, the procedure to open doors has been the same since FSX (and even before FSX ) was released, GSX doesn't change anything of that, it's just that before GSX, not many users had many reasons to open doors other than the main one...
To open a door in FSX, and to use ANY command that requires an initial key press and then a 1, 2, 3, etc. you must first press the initial key (SHIFT+E for the doors), and then, after a very brief PAUSE, a number on the main keyboard (NOT the small numeric keypad).
You simply have to get your timing right...
And, to open the main door, you don't need to press Shift+E and then 1. Just SHIFT+E. The additional number is required only for doors 2 to 4.