I suggest asking on Aerosoft forum, because they programmed the GSX integration and I'm not fully sure of the whole procedure. However, something in your report is off:
Wenn I request the fueling truck it stays in position and ask how much fuel to refuel in %...
When the airplane is flagged as using a custom fuel system ( and the Aerosoft Airbus surely is ), you are NOT supposed to be asked how much fuel % you want. That message will only appear if the airplane is flagged as using the default fuel page and should be replaced with a message "Please use the refueling system of your airplane" instead.
So, it looks like your GSX configuration for the Aerosoft Airbus has a problem, because GSX is not using the one that comes with it internally. Check the airplane configuration page, and be sure is set to internal configuration.