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.
Crawley and surrounding areas are growing anyway with or without Gatwick.
The current thousands of kids who will reach 18 and leave school in the areas around Gatwick will need jobs and wonder where they will come from.
Crawley was fairly major manufacturing in Manor Royal 30 years ago, drive through it now and its offices or empty areas which are used for airport parking.
GACC don't of course attempt to address in any way where all the new jobs are going to come from for even the existing population never mind the natural increase that will happy, rather its a case of No. No. No.
I am fully in favour of a new runway as can see the economic benefits it will bring to the area. Where as the Nimbys have all got to an age where their house prices are of more concern than jobs for people.