We explained the procedure so many times, and is really easy, provided you keep "Detect custom aircraft system refueling" DISABLED ( DEFAULT ) and "Always refuel Progressively" Enabled (this is only to see the GSX fuel counter going slowly up, instead of very fast ):
1) Start with some initial quantity in the airplane.
2) Call the GSX Fuel
3) Don't change the Fuel quantity until AFTER the Fuel truck is in position and the voice asks you to do so.
4) Go in the Fuel menu in the FMC and set the new quantity. If it's higher than the quantity you had in Step #1, the GSX fuel truck will work correctly, otherwise it will go away immediately.