If you are referring to dialog box you see when the Fuel truck comes, is the DEFAULT one from FSX, GSX is only calling it automatically for you, but the units of measure you see there, are not coming from GSX, they are set using the FSX international settings.
The only place in GSX where gallons are used, is the time preference settings, but is not something you use very often, I guess you can simply do that conversion manually when you need to set your timings, once.
