As a general rule, you should take the train to Gatwick if you can, but this is reasonably priced and next time I'm flying from Gatwick I'll probably drive and do this - the train route for me is a bit fiddly, and the worst bit of any London airport for a local is long-stay parking. I'm much closer to Heathrow but the flights from LGW are so much cheaper, it more than covers two weeks of parking at this price.