That's because you either:
- Haven't updated GSX to the latest version, which includes internal support for the Microsoft ATR.
- Downloaded a GSX airplane profile for the Microsoft ATR which is not correct.
- Made some edits to the airplane yourself, without setting the doors correctly.
In all cases, the issue can be fixed by running the FSDT Live Update to get the current version, which has internal support for the Microsoft ATR, and entering the "Configure Airplane" option, and press the RESET button there, to be sure all customization will be discarded, and GSX will only use its internal data, which is surely correct.