GSX obviously works perfectly fine, and is not supposed to do door automation with the PMDG 777, so what you are seeing must be something done by PMDG.
Note that, there's a problem that a variable is missing that could allow GSX to detect the Main Cargo door, which they have added for the 777 Passengers, but I'm not sure if they have for the Freighter version, and this would cause double triggering some services, because GSX has no way to know the initial state of the Main Cargo door.