HeathrowToGatwick.co.uk

The only direct transfer link from Heathrow to Gatwick airport is by National Express coaches. There are about 100 coaches a day between the two airports and it takes aproximately one hour. The coach service is almost a 24x7 operation. At Heathrow airport coaches start from bus stations at terminals 1, 2 and 3. Also coaches visit terminal 4 on their way to Gatwick. Tickets for all coaches can be bought from the travel centre at Heathrow central bus station.
The routes that transfer you from Gatwick to Heathrow airport are the followings:

>>200
>>201 HEATHROW AIRPORT, central bus station > London, Heathrow Airport, Terminal 4 > Gatwick Airport, North Terminal > GATWICK AIRPORT, South Terminal
>>205
>>210 as 201
>>222
>>230 as 201
>>240 as 201
>>700 every 15-20 minutes in the morning. After 12:00 the service operates every 30 minutes until 22:30
>>707 as 201 run half hourly
>>717 as 201 run half hourly
>>727
>>747 half hourly
>>797 half hourly

National Express

National Express is the biggest coach service provider in the UK. If you're travelling to Bristol, Northampton or the West Midlands or anywhere else in UK National Express is the most direct method of transportation from Heathrow Airport.
Visit the National Express information

Oxford Bus

Oxford Bus operate an air-conditioned service to Oxford from Heathrow airport. Coaches depart from central bus station and terminal 5. The travel time is an estimated 90 minutes.
Oxford Bus (The Airline) information