Train
The best connection to central London is the train. The station is at the South Terminal so you may need to take the shuttle to get there. The railway station seems to have been a building site for as long as we can remember but it will surely one day be lovely.
Trains towards London may possibly start at Gatwick, but usually they come up from the south coast, Brighton, Portsmouth, Chichester and similar. Also be aware that Gatwick station has many platforms, and trains go to other destinations, so double check.
There is a good frequency of trains. Indeed, sometimes there can be more than one train at the station heading north.
As it is so far out, Oyster cards are not accepted at this station.
There are three train services to London you can consider:
Gatwick Express
Gatwick Express is a fast service to Victoria. It runs roughly twice and hour during the day, starting when the first planes of the day arrive sometime after 5am.
You can book in advance and it is roughly £20. The journey takes about 30 minutes.
Victoria is an area of London with many hotels, or there are other methods (eg taxi, bus, Underground) to go further. The station itself is huge and uncomfortably busy.
Southern trains
Follows the same route as the Express but stops at many more stations, including the big interchange that is Clapham Junction, so it takes a bit longer.
It’s also a little cheaper, maybe £13.
You are more likely to be jammed in with commuters.
Between this and the Express, there’s not much difference. Southern probably wins for its frequency, and Express for the modern trains and extra space for bags.
Thameslink
Thameslink initially follows the same route as the other services, stopping at many stations, but then diverts east at East Croydon. That may suit your travel better.
It’s a very regular service and similar prices to Southern, but it’s a regular commuter train and can be busy. The seats are jolly uncomfortable.
It goes across the south of London to London Bridge, Blackfriars and then St Pancras, all good for connections, before going north to Luton airport, or Cambridge or Peterborough.