- GSX popps up a window where I can choose different fillings incl. Simbrief Recommendation (this is wrong apparently - it should ask me to enter the % into FMC but GSX DOES NOT aks this)
That's precisely the problem.
The PMDG 737 in the GSX Internal database is flagged has having a custom Fuel system, which means the "
Show MSFS Fuel and Cargo dialog during refueling" option is
Disabled, and it MUST stay Disabled, because its fuel quantity CANNOT be changed by GSX, since it's under control of the airplane own code, that's why we set it as Disabled.
Since you must have edited it and set back to Enabled, you are now seeing the GSX Fuel pop-up window, which you are NOT supposed to see, so the procedure expects GSX to be able to fill the airplane, but because it can't, nothing will work.
Just set the option back to our internal Default ( DISABLED ), or RESET the airplane configuration entirely, so GSX will use only the internal data, which it's guaranteed to be correct.