If you’re new to London, follow “Trains to Gatwick Airport” signs at Victoria, London Bridge, or St Pancras

https://cesarfjap407.theburnward.com/gatwick-airport-travel-times-what-to-expect-from-different-routes-1

If you’re new to London, follow “Trains to Gatwick Airport” signs at Victoria, London Bridge, or St Pancras. Ticket machines and staff can assist with route and fares.