no pun intended as GSX has a mind of its own at times LOL...
The reason of your problem it's precisely because it doesn't have one. If GSX really had a mind of its own, it might be able to automatically recognize any airplane out there without any help from humans. We are still not that advanced and, since that airplane is likely not already internally recognized, and whoever made it likely hasn't supplied it with a GSX.CFG file, you must customize it yourself, setting all the door's positions and setting.
The process is explained on the GSX manual, starting at Page 52, the "Airplane Customization" section.