I have no experience with Gatwick however I bet The second runway there can be used to increase capacity. At such airport where parallel rwys are too close to allow for simultaneous approaches, One is used for departure and other for arrivals. This increases capacity because it allows for one aircraft to line up while the other is still on approach. As the landing aircraft touches down, the departing aircraft can commence takeoff.
A little research before posting this would have removed the need for you to post the comment. Put plainly, that doesn't happen at LGW.