but GSX tank car don't start to fill up. I am always asked to use the refueling system.
This has been explained so many times, on the manual, on the forum, that I probably lost count: when an airplane is flagged as using a custom fuel system ( and the PMDG surely are ),
GSX is not "filling" anything.
In this mode, GSX will only WAIT for the airplane to start refilling ITSELF, using whatever method the airplane use to refill itself and, if a fuel increase is detected, the GSX fuel counter will go up.
The GSX manual has a clear procedure to follow, using precisely the PMDG 777 as an example, and it clearly says you must use the airplane FMC to load fuel
AFTER you see the message from GSX "Please use the aircraft refueling system". If you ( or the airplane or another external utility like a fuel loader or anything that has an effect to the fuel quantity ), change the fuel quantity BEFORE you see that message from GSX, that change won't be detected, so the GSX fuel truck will continue to wait because, the only fuel changes it detects, are those happening AFTER you see that message.