This won't give you much specific help, but 17 years ago I had an 8 hour layover at Gatwick on my way to Riga Latvia. We took the Gatwick Express to Victoria Station. Then we spent several hours on foot (and the tube) walking past some of the major centrally located sites and getting some good pub food before taking the train back to Gatwick. Obviously, this happened back when airport security wasn't quite the chore it is today. I would have liked to have more time, but it was still worth it since I'd never been to London before - or since. If I were you I'd make the effort to get into town for a while. Enjoy!