Is it possible to open all the doors of Airbus A320?
It's not obviously possible to open a door that doesn't exists in the airplane.
Otherwise Catering Service does not work!
That's not the case. Since the front catering door in the Asobo A320 doesn't exist, the default configuration that comes with GSX is to have the door defined in its position, but with the same code as the rear door, meaning the catering service won't be stopped waiting for the non-existing door to open, since opening the rear one will let GSX assume both are opened.
Of course, since the door is not animated, you'll see the catering crew pass through it. If you don't like that, just remove the front door using the GSX airplane configuration page.
In both cases, the catering service won't be stopped.