I just encountered this issue with the A300-600RF PW (IniBuilds FedEx livery) in MSFS 2020 at KSYR (w/ the SierraSim KSYR scenery). Parked at FedEx ramp 1, which is a naturally downward sloping spot where the aircraft will naturally roll when neither chocks or brakes are set. However, when starting a new flight from cold & dark with both chocks and brakes set, the aircraft slowing begins to roll forward until it rolls about halfway into the building. I have not found a way to stop it. This seems to only happen in combination with GSX. If I deactivate GSX the problem does not occur.