From the Gatwick Airport to London, you can get by buses from National Express and EasyBus. The fastest option is the railway express. There are options:

• Gatwick Express to Victoria Station. Departure every 15 minutes, travel time - 30 minutes
• Southern to London Bridge or Victoria stations. Departure every 20 minutes, travel time is 40 minutes
• First Capital Connect to London Bridge, Blackfriars, City, Farringdon, St Pancras (King's Cross) and Luton Airport. Departure every 15 minutes, travel time to London is 40 minutes

It is better to get to London Bridge station then to get to the Old Street station without subways (subway)