In P3Dv4 in the Fuel and Payload screen it does show separate weights for Forward and Aft Cargo and for three sections of Coach, Business, Crew and Pilots. Can GSX not get the total of passengers from there?
And that's exactly what GSX does but, as Kevin already explained, there's no unambigous way to know what a payload station represent (like a "type" parameter), so it can be anything, from a single person, to a combination of many passengers+cargo. The name can be anything, so GSX cannot reliably tell what a payload station represent.
That's why GSX calls that an "estimate" of the passenger number, and that's why you can turn the option Off, and put the exact number of passenger yourself.
And, GSX also has a documented set of variables that airplane developers can use to communicate the passenger number themselves to GSX, which is what several developers like FS Labs, Aerosoft, Leonardo, etc. are doing, so you won't have to manually confirm the passenger number the plane has already calculated by itself.