if I tell the aircraft that it's going to being boarding 180 passengers, and the GSX product loads up to 180 (thinking it's full), but the Airbus says that it has only got to 150, then the door get closed, because GSX has reached the limit... Why else would the aircraft close it's doors when it knows it's not full - an app external to the aircraft must be closing the doors, since the aircraft knows it hasn't reached it's 180 figure, yet.
That's NOT an indication that something external to the Aircraft is doing that. It's only an indication the aircraft doesn't know GSX will be loading 180 passengers, so it's closing the doors because its own internal number is 150.
The way this is supposed to work, when an aircraft interfaces to GSX directly, is the aircraft itself should SET a special variable (documented in the GSX manual), to tell GSX "I'm going to tell you how many passengers me, the aircraft, I think I have". Is this variable is set, GSX will stop estimating the passenger numbers in his own way, and will just trust what the airplane said. If you have the "Estimate passenger number" option disabled, you'll still show you the number, which you are supposed to confirm without changes.
Because, of course, if the airplane said he have 150 passenger, it communicated that number to GSX, which presented you with 150 to confirm, and you *changed* to 180, that will of course screw up any code in the airplane itself that used the strategy of waiting for the last passenger to close the doors, since the airplane was sure to have said to GSX having 150 passengers, we haven't set a system in place were the airplane tells a number, and GSX would send it back *changed* (forcing the airplane to update its count), because the user changed it when asked to confirm.
But of course, that's just an assumption, since I don't know if the airplane is setting the passenger number to GSX, don't know if you had the Estimate passenger number disabled AND you changed the number suggested by GSX (through the airplane, assuming it does that), but that's some kind of problem it can surely happen.