GSX won't change the fuel quantity on airplane with a custom fuel system, let alone the passengers number, which is always a custom internal system. Even planes using a standard fuel system usually have their own internal simulations running on it, so GSX is correctly leaving them alone.
Several developers have coded GSX integration, so their own custom system can work with GSX in the safest way only the airplane developer can possibly know. In a specific case (PMDG), the airplane has an SDK, which GSX uses to program the FMC (so it's still not really changing the fuel/payload: it's just pushing buttons on the FMC) to achieve a similar result, but we used that method because when we added it, PMDG didn't had an EFB, so it was the only way to automatically sync Simbrief with it.
But the best way is to have the airplane developer code an integration, since it would work best with the way the airplane own procedures, that nobody other than the developer knows.