GSX supports only 2 cargo doors in total so, what you need to do, if really want to use that door, is to move another one, like cargo 2 for example, and configure it to use the PMDG variable assigned to the bulk cargo door, because PMDG doesn't use the standard door variables. So you'll need a custom XML expression for that, similar to the one the cargo 2 is already using, but with a different L: variable name.
