Dunno whereabouts in London you're aiming to get to, but National Rail Enquiries shows also a train from Gatwick to St Pancras at 21 minutes past every hour. Stops at East Croydon, Blackfriars and St. Pancras.
Lyon Night
It's Alright :thumbsup:
You could always get a National Express coach from Gatwick back to London. They run all night. Cheap as Chula Chips. Need to book an e-ticket before you get on the bus tho.