You can buy tickets for any rail service at the station or the rail ticket desk in the Onward Travel area in South Terminal. Some airlines sell Gatwick Express tickets on-board, so you can skip the queues once you arrive. You can also buy Gatwick Express tickets from the Moneycorp bureaux and Airport Concierge desks across the airport, as well as Gatwick Connects desks in the baggage reclaim halls.
You can skip the queues by paying for your rail travel with by Oyster or Contactless card by tapping in at the station, with fares from as low as £8 in off-peak periods. You can buy Oyster cards from the station or from the Transport for London desks in both terminals.