The second biggest of London's airports (after Heathrow Airport), though not really in London (it's out to the south in the commuter belt), Gatwick has two terminals (the north is primarily for British Airways, the south for budget and charter airlines) at the eastern end of the two runways (08R/26L at 10364 feet, and 08L/26R at 8415 feet, which is used mostly for lighter aircraft).
Getting from Central London to Gatwick
Gatwick is not on the Tube, but has its own station; Most people travel by rail to Victoria Station by Gatwick Express or local trains. Both are very frequent and the commuter trains run a 24x7 service. The local train takes about ten minutes longer, doesn't have extra luggage space, but costs about 2/3 of the price of the Gatwick Express.
There is also a useful hourly local train to Kensington Olympia and West Brompton.
The cheapest London transfer of all is the National Express coach that runs broadly an hourly service to Victoria coach station, but takes twice the time of the train.
Getting from Gatwick to Luton, Heathrow and Stansted
National Express operate a Gatwick to Heathrow bus shuttle. There are around 100 services a day each way!
Thameslink operate a direct train service to Luton , several times an hour. Though you use a free bus shuttle between Luton Parkway station and Luton Airport.
Although National Express operate coach services between Gatwick and Stansted Airport, indirect routes via Central London using the Gatwick and Stansted Express interconnecting using the London Underground is faster and more reliable.