The default configuration for the PMDG 737 that comes with GSX surely uses only Conveyor Belt, no doubt about that so, unless you change it yourself, you shouldn't normally see those.
However, it's possible if you are using a custom GSX profile for the airport, that whoever made it changed the loader type at that parking spot, probably because he doesn't use 737s.
Usually the loader type doesn't require changing on a parking spot, especially if the parking is suitable for non-Heavy airplanes. The reason for having the option is usually to *disable* a loader because it couldn't maneuver in any way without clashing into something, but for the maximum flexibility we also allowed to set a specific kind, even if it shouldn't probably used, unless you are sure that parking spot is only used by planes that *can* use an ULD loader.