I strongly recommend reading the manual better, which explains that:
- The passenger number is automatically ESTIMATED based on the weight of the payload stations, divided by 220 lbs.
- Which means, you must configure the payload *BEFORE* you call GSX. If you call GSX with an empty plane, GSX will assume the plane is empty. No, GSX won't touch your airplane payload, since most airplanes have their own procedures/loaders. GSX will, instead, adapt to your existing payload, the moment you call it.
- There's an option called "Estimated passenger number", which you can turn off, so you can set the number of passengers directly, because the automatic calculation cannot always work, both because some airplanes change their payload in realtime, but also because the ratio between cargo/passenger is not known, so the 220 lbs is only an imprecise aproximation.