If the airport hotels aren't working for you (unless you're able to walk, none will offer free transport AIUI) you might want to check out options in Croydon. It's half way between London and Gatwick. It's also in zone 5 of the London travelcard area so you can get that far with an Oystercard, then buy a paper ticket for the rest of the way the next morning.

I see there's also a Travelodge opposite Redhill Station - again many of the London to Gatwick trains stop there.