Pretty sure PMDG provided a profile.
On my system, in the community folder there is a GSX profile at:
..\pmdg-aircraft-77er\SimObjects\Airplanes\PMDG 777-200ER\presets\pmdg\PMDG 777-200ER GE\config
Looks like they have them by engine variant. What's not working? GSX seems to find / use it.