If no SimBrief then how to change the passenger number?
A method could be making a small LUA macro with FSUIPC or LINDA to set the L: variable GSX expects, as if you were an airplane. The last page in GSX manual are not just for developers, using such utilities, the information is also useful for users.
I'd like to stress that, having to manually insert the passenger number was a chore, and it's far more realistic to use a Dispatch system, because surely the pilots (you) are not supposed to tell the ground crew how many people to board so, if you are really interested in having the *accurate* number of passengers, indicating an interest to realistic operation, why not just using SimBrief ?
If you are just pressed for just a quick flight, does it really matters in that case if the number of passengers automatically estimated from the payload might be a bit off ?