Updating Couatl won't fix this if you have an old version of the GSX vehicles so, first, download and install the current full installer for GSX, to be sure you have all the updated vehicles too.
If this doesn't change it, it means you have a custom configuration for the airplane, which is taking precedence (this is normal), to the one that comes with GSX, likely because you tried to configure the airplane manually, and made some kind of mistake. To verify this, remove any GSX.CFG file you might have in one of these folders:
%APPDATA%\Virtuali\GSX\Airplanes\PMDG 777-300ER
FSX\Simobjects\Airplanes\PMDG 777-300ER