Is there a tutorial for the editor?
Yes, of course, in the GSX manual.
I thought it worked by placing the ground vehicles where I want them for the doors and what not.
I think you are confusing the *parking* editor with the *airplane* editor.
The one that places the vehicles interactively is the *parking* customization, and is used to decide the STARTING positions of the vehicles on the apron, in order to better tune GSX to an airport and be sure those vehicles won't start, for example, inside a building or over an object.
The one that you need to use to adapt an airplane, is the *airplane* configuration editor, which adapts GSX to any airplane, and there you places ONLY doors, vehicles are not even displayed there.
The airplane graphical editor is explained at Page 31 of the manual.