Hello MWcw709!
I followed the GSX recommended settings from the manual:
1. Always refuel progressively as ENABLE
2. Detect custom aircraft system refueling as DISABLE and
3. Assistance services auto mode as DISABLE.
I really think in my case the GSX is inserting the amount of fuel too early (just as the fuel truck arrives).
When I’m not using the simbrief integration, it asks to insert the fuel some seconds after the fuel arrive (a bigger time delay between the fuel truck’s arrival and request to insert the fuel amount) and then it works just fine!
I hope this helps Umberto to help us!
BTW, the same problem is being experienced by my friend.
Cheers!