The Marshaller will stay on a ladder IF the program assume there MIGHT be a building behind him, so you might not see him. If the program assume there will be enough space behind him, he'll be on ground.
The assumption is based on:
- The position of the eyepoint in the cockpit
- The parking radius
- Whether the parking has a jetway or not. This might be an issue if the 3rd party scenery you use doesn't have proper FSX jetways, so GSX cannot possibly know there is a jetway there, so it will assume it's a parking in the open.
Of course, this won't affect JUST the marshaller ladder, but also the appearance of different ground vehicles (the passenger bus and the front stairs won't appear on a parking with a jetway)
That's why, if you believe the 3rd party scenery you use doesn't have proper FSX jetways, you must first configure all parkings that have fake/non-standard jetways, to indicate there's one, since GSX cannot otherwise detect them automatically, which is what it does when the scenery has proper FSX jetways.