1. Make a folder in C>Users>myname>AppData>Roaming>Virtuali>Aircraft named B777 200LR
There's no such thing as an "Aircraft" folder in GSX. If you never customized an airplane using the GSX editor before, there's not folder at all, but if you have, you'll find a "Airplanes" folder, with several sub-folders, each named like the airplane you customized.
So, if the "Airplanes" folder is not there already, you must create it, and it must be named like this. "Aircraft" won't work. We used "Airplanes", because this is how the Airplanes folder is named under Simobjects.
2. Downloaded a aircraft.cfg file from the sharing area for the B777-200LR
The aircraft.cfg file is the file the simulator use to configure an airplane, it doesn't have anything to do with GSX. A GSX airplane configuration file is named GSX.CFG.
3. Place the .cfg file in the B777 200LR folder
You can place a GSX.CFG file in the airplane own folder ( the one that contains the airplane.cfg ) or in the folder you created under %APPDATA%\Virtuali\Airplanes\AIRPLANE_NAME. If you use %APPDATA% ( which is the suggested way ), the only thing that matters is the airplane name folder matches exactly the airplane own folder name under Simobjects\Airplanes
