Sorry to be a wet blanket but I am having the same issue with the default 737-800.
No, you are not. The issue with the PMDG was that, by adding a code to a door, will lose the internal customization for that airplane that, in case of the PMDG is made with a custom variable.
Door 3 will not open for catering after update 1.4.
Please explain better what you are saying: If the door 3 doesn't OPEN on your airplane (which is seems to be the problem, reading your message verbatim), this doesn't obviously have *anything* to do with GSX.
In fact, by default, the Exit 3 on the default 737-800, with the aircraft.cfg as supplied by Microsoft, DOES NOT OPEN AT ALL. It's the GSX installer that FIX this, by changing the following (WRONG) line in the original aircraft.cfg of the 737-800:
number_of_exits=2
INTO THIS
number_of_exits=3
This control if you can OPEN the door or not in FSX. If you restored the original default aircraft.cfg for some reason, you'll lose the FIX that was made by the GSX installer.
This concludes actual doors OPERATION.
What the GSX.CFG file controls is NOT the door OPERATION. Is how GSX REACTS to the door's status (open/closed) or it's very presence.
There are no problems in your GSX.CFG, other than it shouldn't be needed at all for the default 737-800, since it's an already fully supported airplane. However, having one should cause any problem, and the code = 3 is correct for the Service doors.
But, as I've said, if your problem is the doors simply doesn't open, that's nothing related to GSX at all: either you haven't properly pressed the keys, or the aircraft.cfg was restored to default.