By default, 24 000 kg of fuel are loaded in the plane on cold and dark, that's why GSX cannot load the plane according to the Simbrief loadsheet if you planned less. An option allows then to load the Simbrief amount of fuel and then the GSX OIS menu is closing. But you have to click on the fuel button to load the plane correctly. Of course, I think this default amount of fuel may be changed somewhere, to get only 10 000 kg on C&D for example. But for long flights, it would take a long time to refuel the plane. 24 000 kg is probably the best compromise...
Same thing for the passengers boarding : it takes 10 mn for GSX to board about 150 pax and I think that's why the boarding sequence is achieved before the total of passengers is reached, if their number exceeds 150 or so. Once the boarding is completed, I click on the pax tab and I get the correct figure.
Perhaps it would useful to add something on GSX integration in the FAQ's page.