I'm flying FSL Airbus and every time I start boarding with GSX, my manually entered payload gets overwritten by loading from GSX.
Whatever you are seeing, it must be some function of the airplane, because GSX doesn't change the airplane payload.
It changes the Fuel, if the airplane is flagged to use the default Fuel system, which is not how we configured the FSL Airbus in the internal GSX configuration but, of course, if you have changed it and set the flag to use the the default Fuel system back on, GSX will refuel the airplane.
But again, even in that case, it would be only the Fuel, not the Payload, which is never changed by GSX.