Please check if you are using the configuration that comes with GSX, not one you made or downloaded, especially if it was made/downloaded before GSX added support for the main cargo door. You can use the GSX airplane configuration page to check which configuration GSX is using. Pressing the Reset button will revert to the default config that comes with GSX.
Also, be sure you use the current GSX version (Live Update), because in the current version, no vehicles will appear for a door that doesn't exits.