Again, which airport (ok, EDDH, which scenery, default or 3rd party ) ?
As I've said (I've just tested it now), with the default 747, if I open the door 1, 2 doors open and 3 staircase appear. To open the other doors on the left side, you need to open the door 3 (service) which opens *all* the remaining doors on left and right side.
The reason why I doubt you are seeing exactly what you described it's because (forgetting the staircases for a moment), it's just not possible you would have 3 doors opening when opening the Exit 1 with the default 747, as you described.
Only the first 2 are linked to the Exit 1 in the default 747, and this doesn't have anything to do with GSX, it's how the default 747 model is made, it would be like this regardless if GSX is installed or not, since GSX doesn't control the airplane doors at all, only the airplane model ( and its gauges) itself can do this.