When the airplane use its own ladder, you MUST use the "Embedded ladder" option in GSX, no questions about it.
To make it work, you must be sure that you set BOTH the door coordinates AND the embedded stairs "start" and "end" point. Also, understand the doors diagram in the GSX airplane customization image is only an example so, in this case, where's the only passenger door is behind wings, it STILL must be the Door 1, because GSX won't show passengers if there isn't any Main passenger exit so, if you use that for Door 4, for example, without a Door 1, no passengers will show.
The airplane picture doesn't have any meaning other than being just an example, the location of the icons there don't have any effect on the actual door locations, the only thing that matters is the actual XYZ coordinates so, if the plane has only one passenger exit on the back, use only Door1 and move it back there with the editor.