I could solve this issue myself. I misread the instructions above and put “ParkingBrake=100.00” in the wrong section. This lead to there being no improvement of the GSX functionality.
To summarize and paraphrase GarciaMK’s instructions:
In file systems.cfg put “parking_brake = 1 ; BrakeTypes enum” in the [BRAKES] section, or change it from “parking_brake = 0”
Meanwhile in all *.FLT files add “ParkingBrake=100.00” under the section [Controls.0] (Not the [Brakes] section as I erronesouly did first). It might not be necessary to change all FLT-files, but it worked for me like that.
Maybe this helps others making it work.