Since there's no such thing as a standard variable for the Passenger Number, any airplane that has its own passenger number simulation, will do it in a custom way so, unless the airplane has a way ( like an SDK ) to control or even just read that number for other add-ons to use, it's something that stays inside the airplane.
GSX can read your flightplan from Simbrief, so at least the number of passengers visible will match the plan. But to get the same number in the airplane, one of the following must be true:
- The airplane has coded GSX integration, the first product out that has this feature, is the Leonardo MD80.
- The airplane has its own Simbrief integration so, even if it doesn't have direct GSX integration, they will match as long the same flight plan is read.
- You set the airplane own Pax number to match Simbrief manually, if the airplane doesn't have Simbrief or GSX integration.