Hi Cindy,
I live in Yorkshire and vary how I get to London Airports between Car and train depending on flight times. I've never actually had a Gatwick flight although a few of the holidays I'm considering in future are from there.
Last year I did Americas Golden West from Heathrow and used the train but this year for Parks & Canyons its the car because I'll be travelling down Sunday and don't trust sunday trains. Train is the most economical if you book your tickets in advance.
I nearly always stop over the night before and usually use my AA membership to book as it gives you discounts on hotel and parking deals. Managed to get the Renaissance on Bath road, hotel and 11 days parking plus breakfast in June for £94 which looks a good deal.
I'll check out the site you suggest could be useful in future.
Thanks
Mick