It's entirely normal, if a 3rd party scenery use a custom Docking system, it's not possible for GSX to know it has one, the only Docking systems recognized by GSX are GSX's ( of course ) and SODE VGDS so, if the scenery uses these, it won't automatically use a Marshaller, since it *knows* there's a Docking system there.
In 3rd party scenery using custom ( non-GSX or non-SODE ) Docking Systems, you must set the "No parking System" option in the GSX customization page for all parking with a custom docking system, which will prevent GSX to create a Marshaller.