There's no need for a config file for the PMDG 747-400, since it's a natively supported airplane in GSX. Nobody ever reported a problem with it so, the only possible explanation is that you tried to do a GSX.CFG for it yourself, and made some error.
Remove any GSX.CFG you have for the PMDG 747-400, either in the %APPDATA%\Virtuali\Airplanes\PMDG747-400 folder, or in the Simobjects\Airplanes\PMDG747-400 and try again, it must work without any GSX.CFG file.