As explained on the GSX manual, the choice between cargo and baggage loaders is decided by the airplane configuration, which you can obviously customize.
If the setting for a cargo door is "Auto", the choice will be automatic depending on the door's height.
If the setting for a cargo door is "Belt", it will *always* be a Belt loader, provided is not too high the Belt loader couldn't reach it
If the setting for a cargo door is one of the ULD codes, it will *always* a Cargo loaded, loading that kind of ULDs.
