If you mean the Leonardo MD, isn't supposed the airplane coming with a pre-made GSX config file in its folder ?
As far as I know, the base version didn't use a custom Parking brake system, meaning the airplane use the standard variable for Parking brakes, so it should work.
- If this variant doesn't use the standard variable for parking brakes anymore, then a custom one should be specified in the GSX.CFG file, but it's strange if they made this change, they didn't added to it.
- If nothing has changed, and the parking brake variable used is the standard one, then the problem doesn't have anything to do with GSX, for some reason the parking brake variable is not set, possibly because of some other system that needs to be set correctly as well ( hydraulic pressure, maybe ? )