The existing emergency runway at Gatwick is too close to the main runway for safe joint operations - therefore a new runway to the south is the only option!
You may also notice that the two runways are too close for safe operation together, 08L/26R is also far too short and is currently non-instrumental. So it is only for emergency use if something happens with 08R/26L! So Gatwick is effectively only a single runway airport. To be considered a two...
Gatwick will also link in with an expanded Thameslink network when London Bridge is finished in 2018 as well as an interchange with Cross Rail at Farringdon.