GSX should automatically recognise the new scenery installed. If that doesn't happen for whatever reason, you can try forcing a complete cache rebuild, by removing the following folders:
C:\ProgramData\Microsoft\FSX\SceneryIndexes
C:\ProgramData\Virtuali\Couatl
And restart FSX, you should see both FSX and GSX rebuilding their scenery caches, the new scenery should work at the end of the process.