Since that plane is not included in the GSX internal configuration, it means you are using one you made so, just be sure you set all doors correctly. It's not very clear which door you are missing, since a 747 cargo has *two* back cargo doors, one on the right and a bigger one on the left side.
The one on the right side it's usually assigned to the cargo door 2, and the one of the left, to the main cargo door.