GSX reads the Afcad file for each airport and based on that information it "knows" if an aircraft is going to fit in a certain position. You do not give any information what YSSY scenery you are using. ( please do so next time). I checked the default YSSY airport and there is only one gate ( 50 ) that has a radius large enough to accomodate a widebody. So based on this afcad GSX will only offer you 1 gate, provided it is empty. The afcad here is therefore wrong. Solution is not too dificult. Download ADE from http://www.airportdesigneditor.co.uk/
and edit the gates that you want to accomodate a widebody to "heavy"
b rdsg Dick