I click to fuel using default FSX fuelling system and nothing happens. The fuel truck just sits there, does nothing and no menu opens.
I think you misunderstood what the message is asking you to do. You haven't "clicked to fuel", the message told you to use the default FSX refueling page, which will be used to tell GSX how much fuel you want so, it's not stuck, it's waiting for you to act on the fuel page and change the quantity. If the quantity doesn't rise, it will wait forever...
Then, when you finally told GSX how much fuel you want, it will refuel progressively.