You can easily add any airplane to GSX using the integrated airplane configuration editor. Since the CS777 uses lots of custom XML variables, the easiest solution would be:
- Load the SUPPORTED variant of the airplane
- Verify it works correctly with GSX.
- Open the Airplane configuration editor in GSX
- Press the "Save" button
- A GSX.CFG file will be created in the %APPDATA%\Virtuali\Airplanes folder, under a sub-folder named as the airplane you loaded
- MOVE this file in the airplane root folder, the one that contains the airplane.cfg, for the unsupported variant.
- Load the new variant in FSX
- If the model is the same (eg. doors are all in the same places), it should work right away. If something has moved, just used the Airplane configuration editor to adjust any positions that might have changed.