The refuel truck works perfectly with the PMDG 777 and it was the one most tested. I tried it just now, and it works normally.
The important thing to remember, is that you must change the fuel quantity on the FMC AFTER you see the message "Please use the refueling system of your airplane", which is also explained in the manual. Only after that message, GSX will monitor any sudden increases in the fuel level. Anything you do before that message, will be ignored.