7 hour layover at Gatwick

I'm in the same position with a 6 1/2 hour layover April, 2016.

You could catch a train into the heart of London, but it's seriously expensive. See: http://www.gatwickexpress.com/en/tickets-and-fares/buy-online-and-save/
You really wouldn't have that much time in the city by the time you take the train two ways. Then you'd have to go through Gatwick security and leave yourself enough leeway to make your flight. It'd hardly be worth it.
We're flying back thru Gatwick (from Venice) returning home, and decided to spend the night at the airport. Our flight leaves mid afternoon which still doesn't give us enough time to go into the city.

You will have to consider you need a couple of hours ? to be at the airport / check in since security is tight .. That said it's a about a half hour into London to Victoria Station on the fast metro that is another hour. You could take a taxi at the station then to Westminster Abby or ride around to see Buckingham Palace.

Their are websites for all of these:

One place I loved to see and I only had 5 or 6 hours and it's not far from Gatwick, " I have used that airport a lot in my travels to England.
We took a taxi from the little village as we stayed in Gatwick before our flight . Hever Castle a jewel of a castle where these two women grew up, Queen Anne and her sister Mary "minstress of Henry the VIII his boy child lived here when she was married to another" . You could spend 3 hours there and another hour in the Maze.. Not on the time you have !
Plus in Gatwick there is a great place to eat "we have eaten here 3 times over the years " , YE Old 6 Bells , "from 1600's " pub right next to an OLD church . There is a cow pasture / stream behind it. All the locals go there to eat. Wonderful food too. I would do this again in a heartbeat.
What ever you do you'll need to use taxi's

Sorry, but 7 hours isn't really long enough to get into London and do much before heading back to the airport. Assuming your flight is on time, it will take you an hour to clear customs, then another hour to get into London. You will have to be back at the airport 2 hours before your flight and it will take you an hour to get back there. That leaves you 2 hours max.

I am not sure if you will have your bags booked through or if you will have to take them with you. London the week before Christmas with luggage will be a nightmare, as it will be busy.

Don't forget that it gets dark here early at the moment - before 4pm.

2 fantastic gardens close to Gatwick ...
http://www.nationaltrust.org.uk/nymans
http://www.nationaltrust.org.uk/wakehurst-place

30 mins on the train to Brighton might be possible - go for a meal and a walk - then train back to Gatwick. London on a Friday afternoon before Christmas will be heaving and best avoided. Decide on the day but give yourself sufficient leeway.

Suggestions borrowed from TA:
http://www.tripadvisor.co.uk/Travel-g186338-c195392/London:United-Kingdom:Heathrow.Layover.Central.London.html

Thank you very much for all the information. It's been incredibly helpful. I am nervous about leaving gatwick now so we shall find things to do within the airport. Thanks again

Thameslink is a company with which I have very close personal knowledge.

It is a good suggestion but does have a few difficulties including its perpetual lateness, sometimes down to causes not of its own making, the absolute pigs breakfast of London Bridge at the moment meaning the trains take a rather slow and circuitous route between Blackfriars and the south, and the rolling stock, partly made up of modern Class 377 trains with air conditioning and comfortable seats with good electronics and announcements, and a fair bit of the fleet still made up of ancient Class 319 (and variants) which don't have any of that.