I know there is many space in a B747-8, but 711 passengers are a little bit too much
Page 11 of the GSX Manual, where it explains how GSX estimate the number of passengers, by default:
GSX will calculate Deboarding time by estimating the number of passengers on board, which equals to the sum of all the stations in the [weight_and_balance] section of
the aircraft.cfg of the airplane in use, divided by 220 lbs. which is an industry standard method of calculating how much a single passenger + baggages weights, in average.
The simulator doesn't make any difference between payload, it can be passengers, or cargo, but it's generic so, with larger airplane with some cargo space, the automatic estimate of passenger numbers will be off, since there's no way to know which stations are passenger and which aren't.
Page 30 of the GSX Manual, which explains the
Estimate Passenger Number option, can help you:
Estimate passengers number
If this option is disabled, GSX will always ask confirmation from the user about the number of passengers onboard. If the option is enabled, GSX will estimate the passenger number automatically, without asking confirmation.