Surely, if Gatwick (or Stansted or Greenham Common or anywhere else) had four runways and everything else then people would "want" to use that airport.
It's not just the airport but the ease of getting to it or getting out to home/work. To expand Gatwick/ Stansted and especially somewhere like Greenham Common would mean spending £bn's on improved transport and then the argument against LHR goes away.
Thameslink has improved Gatwick to some extent but it will never compete with the future connectivity to LHR.