Many people on here have used http://www.holidayextras.co.uk they will bring up various options for you, although I couldn't guarantee that this is the chepaest way to do it. I have booked various things through them car parking with an overnight in a gatwick hotel and just normal airport car parking as well. I guess its worth checking their price against booking direct the the one you finally settle on. I tend to use them because its an easy option. and I've had no probs so far.

the last few years we have stayed at the Cambridge hotel only 10 minutes away from airport never had a problem parking and room for the night £80 (was last year ) i know it was taken over last year so maybe prices will be higher the owner had a lot of plans for the place

much of the info you are looking for can be found in the flights/airports forum Gatwick Q&A's

wizard