Hi Ben, the session files would just be for the purpose of backing up.
To export the song as a whole you would have to make a performance recording first in GTL. Then you could export your it via iTunes file sharing. I think you would have to drag the GTL 'Performance Recordings' folder over to your desktop first and then drag the files into iTunes.
I will look into direct export to your iTunes library but it's not possible at the moment I'm afraid.
Hope this helps.