From your question, it seems you haven't read the refueling pages in manual. On an airplane that has its own custom refueling system, GSX doesn't refuel at all, it only play animations, related to the changes in fuel quantities made by the *airplane*.
Please read the two examples at Page 16-17, which explains precisely how you are supposed to use it on a 3rd party airplane with a custom fuel system, and the differences if it has a progressive refuel simulation or not.
But even on an airplane that use the standard fuel system, GSX will never de-fuel. If you have a certain quantity of fuel, and you set a new one which is lower than the one you started with, it won't do anything.