By road London's Heathrow and Gatwick airports are about 40 miles apart.

National Express Coach Service
The only direct transfer link is by National Express coaches, there is no direct rail link. There are around 100 coaches a day between Heathrow and Gatwick taking around an hour if there is no traffic congestion. The coach service is almost a 24x7 operation.
At Gatwick the coaches pick up from each terminal, at Heathrow the coaches start at the bus station between Terminals 1,2 and 3, they do not pick up or set down from each terminal at Heathrow. The coaches visit Terminal 5 at Heathrow on their way between Heathrow and Gatwick. If you are using Terminal 4, you need to catch the free Heathrow Connect train that runs every 15 minutes to the central area where the bus station is. There are luggage trolley friendly underground tunnels between Terminals 1,2 and 3 and the central bus station. Allow 10 minutes from exiting customs. The ticket office at Heathrow Bus Station can have very long queues at peak times.
National Express coaches allow a maximum of 2 medium size suitcases. Folding bikes, surfboards etc are carried only at the discretion of the driver. Bikes must be packaged like that required by the airlines. No animals except guide dogs.

Time to allow for a transfer:
At both Heathrow and Gatwick it takes on average about 45-75 minutes from landing to clearing customs with luggage.
The route is all motorway, but a very congested motorway where delays are very common. Without traffic congestion the National Express  will take around an hour to complete the journey. Very few trips during day time will not experience some level of traffic congestion. Journeys between 07:00 and 10:00 and 15:30 and 19:00 will routinely take 90 minutes or longer.

At Heathrow Terminals 1,2 and 3 allow 15 minutes to make your way between the terminals to the bus station.
For Heathrow Terminal 4 allow 20-30 minutes between Terminal 4 and the Bus Station via the Heathrow Connect train.
At Terminal 5, the buses pull up in front of the terminal.
At Gatwick, the buses visit both terminals.

Cost
The one-way fare is very high considering the 1 hour journey time perhaps reflecting the monopoly position. The cheapest route between Heathrow and Gatwick is not direct, but uses the rail link from Gatwick Airport station to East Croydon (takes 15 minutes, frequent service, costs about £5; ask for a Southern ticket, the other operator charges more).  Just outside East Croydon station is the end of the run for the X26 Metrobus (timetable online at www.metrobus.co.uk ) which takes between one hour and 20 minutes and about two hours to reach Heathrow, at a cost of £2 single and is free for children under age 11. Another way,  more arduous,  is to use the London Underground between Heathrow and London Victoria and bus or train between Victoria and Gatwick. This is much slower though and the London Underground is very crowded, especially uncomfortable with luggage or coming off a long flight. The cheapest underground/bus option will commonly take 3.5 hours airport to airport.

If there are more than 5 people travelling the cheapest option between Heathrow & Gatwick is private hire minibus (see below).

Licensed London Taxi Cab, Private Hire and Mini Bus Options
The fastest route is by taxi or minibus. Without traffic congestion the fastest transfer can be achieved in around 45 minutes.
If there are 6 or more travelling it is actually cheaper than using the direct National Express bus.
The London black taxi cabs on the rank in front of the terminal at Heathrow will go to Gatwick, both 5 and 6 seaters are available and most will accept credit cards, a price can be agreed with the driver at the comencement of the journey,

You will get the best price by booking in advance. Competition is fierce and advertised headline rates by taxi companies can be misleading. The most common extras to look for in the small print are:
- surcharges for paying by anything other than cash.
- car parking and 'meet and greet' charges
- waiting time (remember the average traveller will take 45-60 minutes to clear customs) many companies start charging after 30 minutes - not enough luggage capacity requiring an upgrade or extra car.

Benchmark companies to compare against for cars include Simply Airports and Just Airports

If you have a small group or outsize luggage FCMBS offer minibuses from 8 to 16 seats capable of carrying at least 16 large cases plus hand luggage. An 8 seat bus currently costs 120 GBP between Heathrow and Gatwick.