Okay, deleted the files, but still the same problem. I see that GSX also doesn't show all the vehicles in the editor. Only the plane and the pushback truck
This problem doesn't have anything to do with another addon stealing the keys. That would only cause the vehicles not moving.
In the editor, you will only see vehicles that can work with your currently loaded airplane. Stairs, for example, won't appear if you use a small Cessna, since that plane will not use any stairs. Or, it can happen if you use the editor with an unsupported airplane that hasn't been configured for GSX yet.
This is not necessarily a problem, since even if you don't see the vehicle itself, you can still customize its position by moving the orange pointers (blue, which will turn to orange when active) but, in general, it's best to customize a parking with a supported airplane, so you can see all the vehicles.