The issue is, Simbrief already works with GSX in FSX/P3D, using the basic functions which are:
- Passenger count taken from Simbrief
- Fuel quantity option to use Simbrief Block fuel in refueling, when airplanes are set to use GSX refueling.
You only need to be sure the airplane ICAO is set correctly in the airplane profile, and it matches the one on Simbrief, because FSX/P3D don't have the "icao_type" parameters in the aircraft.cfg, so you usually need to set it manually in the airplane configuration page.