On an airplane flagged as using a custom refuel system, like the PMDG 737, you must be sure to follow this method:
- Call the GSX Fuel Truck
- Wait for the message "Please use the refueling system of your aircraft to refuel"
- Load the fuel from the 737 FMC menu
Now, the time it will take for the GSX refuel, will change depending if you set the "Always refuel progressively" option in the GSX Settings. If the option is disabled, the GSX refuel will be very fast, but if it's enabled, it will take the same time it would have, if this was a default airplane that would have been refueled progressively by GSX, according to the fuel truck pump speed specification and your "Fuel Time Acceleration" setting in GSX.