It would have been best if you just posted a log to confirm the problem, that would surely take way less time than writing a program assuming GSX has a bug.
The number of passengers is correctly read by GSX in FSX and P3D too, the most likely issue is that, the flightplan wasn't accepted, because the airplane type wasn't recognized. This because Simbrief uses the ICAO type designator in the flightplan, and FSX/P3D don't support the icao_type_designator in the aircraft.cfg, which is a new setting that has been introduced in MSFS.
GSX allows you to set it in the airplane configuration page so, even if it's not supported by the FSX/P3D aircraft.cfg, it can be set in the GSX airplane profile, so it would work.
Of course, I just tested it with the A321 imported from FSX, by default the passengers number wasn't set ( the GSX log would say "can't read the airplane type" ), but as soon as I entered A321 as the Icao type in the airplane configuration page and restarted GSX, a Simbrief flight plan for an A321 was read, and the correct number of passengers was set.