The GSX manual includes two usage cases, using precisely two different PMDG models ( 777 and 737 ) as an example.
Is it possible to change it to kgs like most European airliners?
The indication in the green text line is already depending on your international settings. What you must deal with (if using the PMDG), is of course dependent on the PMDG FMC settings.
The thing that is always in gallons, is the fuel counter indicator over the vehicle.. Maybe we could change this, but I guess it should be more related to the airport, rather than following the sim preferences.