Given the latest Questions about this, I would also recommend making the Crew Boarding a Central Option in the Settings GUI.
(And maybe in the long-term making it a separate Operation/Service - the Crew does Board the Plane, but that does happen as very first "Stept" - not together with the Passengers^^)
BUT: That doesn't mean it can't be disabled!
Put the following Lines in a Lua-Script (registered Copy of FSUIPC required) and bind it to a Key/Button with FSUIPC.
Then just press the Button/Key before (De)Boarding but after GSX is activated on the Stand (Vehicles showing up).
ipc.writeLvar("FSDT_GSX_PILOTS_NOT_BOARDING", 1)
ipc.writeLvar("FSDT_GSX_CREW_NOT_BOARDING", 1)
ipc.writeLvar("FSDT_GSX_PILOTS_NOT_DEBOARDING", 1)
ipc.writeLvar("FSDT_GSX_CREW_NOT_DEBOARDING", 1)
ipc.writeLvar("FSDT_GSX_NUMCREW", 0)
ipc.writeLvar("FSDT_GSX_NUMPILOTS", 0)