I know how to change in P3D Kg and Gallons.
GSX allways Show me at the Refuling Truck Gallons no KG or Liters.
The counter on the Truck is always in Gallon. What changes is the green indication at the top.
But that's not really an issue. It's not that GSX is asking to *enter* a Fuel quantity in an unit of measure you are not accustomed to. With the PMDG 737, the unit of measure to *enter* a fuel quantity depends on its own settings. GSX will only show a progress indication, in two places (on the fuel truck and on the green bar on top), and the one on top DOES indicate the progress depending on your International settings.
Every Time a Change something in GSX i restart couatl nothing changes at the Progress.
I can only repeat and confirm that, with the Always Refuel Progressively option, the refueling time is realistic. Of course, as explained on the manual, this is also affected by the Fuel Time Acceleration slider in the GSX Settings.