Second rwy doesn't address the real issue: no spare capacity at the UK's hub airport.
Gatwick is in a semi rural area with very low unemployment, so where do the extra workers come from? outside the area.
Therefore they'll need houses and other infrastructure, this means urban expansion, and more people affected by noise than is accounted for.
This happened when LHR expanded in the 1950s post-war recovery with many moving (from Glamorgan and around Bristol in particular) to Middlesex to work on the airport and this lead to the expansion of former villages like Stanwell, Cranford, etc. into towns.