*[Before January 2016]:* You can get an Oyster card from the airport.  However, it's not available on the trains yet.  You'll have to buy a ticket at the station (there are machines on your right as you walk into the train station at the airport).

If you want to go to Monument, you'll only have to change once, and you won't need to buy a new ticket. Buy a ticket at Gatwick to Monument - it'll be a Day Single ticket.  Catch the train to London Bridge, and once there, go down to the tube station (you'll see signs, so many signs) and get the Northern Line northbound on platform 1 to Bank.  Bank and Monument are interlinked.  From there you can walk to Old Billingsgate.

However, if you prefer a scenic walk - and it probably won't take you any longer and will save you some cash, as you can see [on this map](https://www.google.com/maps/place/Old+Billingsgate/@51.5094946,-0.0884528,15z/data=!4m2!3m1!1s0x4876035197ad1801:0xfbd34dc4f1fbc6fb), London Bridge station is pretty close.  If you buy a ticket there, you can exit the station and follow the signs to London Bridge, and walk across London Bridge.  As you do so, look to your right and you'll see the famous Tower Bridge as well as the Tower of London.  It's a pretty great walk.




  [1]: http://www.gatwick-airport-guide.co.uk/london.html