GSX surely recognize all doors in the PMDG 737 provided, of course, you use the default configuration that comes with GSX (which we know is correct), because if you tried to customize it and make some kind of error, we cannot possibly know.
So, the first thing to check for, is if GSX is using its own internal database, which can be verified with the Airplane configuration editor. If it doesn't say "internal database", use the RESET button to discard any custom configuration you might have made/downloaded, and RESTART COUATL from the menu.
If GSX still doesn't seem to recognize the airplane doors in this case, try to close them and open them again. We had some reports that is some cases, the PMDG 737 doesn't initialized the variables until the doors have been used at least once.