i opened airport EGNX with a 747-800 and located in a large parking area .comments no gsx sevices parking to small for aircraft..
If GSX says the parking is too small, then IT IS to small, regardless how visually looks like, it's too small in the AFCAD.
You can override the value found in the AFACD using the Parking editor in GSX, this is all explained in the manual.