For posterity... per the GSX Manual:

An airplane can be customized by creating a file named GSX.CFG, that needs to be
placed in the airplane Root folder, the same that contains the aircraft.cfg file. For
example, if you want to customize the default Boeing 747-400, you will have to create
a GSX.CFG file in the FSX\SimObjects\Airplanes\B747_400 folder.

In my sim, the gsx.cfg file goes in:
E:\Program Files\Lockheed Martin\Prepar3D v4\SimObjects\Airplanes\CS_B767-300ER-II_PW4060