Keep in mind this growing market, but the new runway goes to Lgw. What do the airlines that need to add capacity do? Walk away? Or the new ones with maybe new routes?
What people tend to do is throw "airlines" together as one mass. They're not. Some will only serve LHR, long haul airlines connecting the UK to the world have been proven time after time after time to do less well at Gatwick. This means their competitors at LHR have them for breakfast and they fail. So to get long haul airlines into the UK, the overwhelming place they choose is LHR. If they can't get in, they add capacity at a partner hub like Skyteam AMS/CDG or STAR FRA/ZRH. This is because in this market, the big boys need to be in an alliance and they stick together. You might say look at Garuda, they serve LGW. They do, and they fly an almost empty B777 out most days except weekends when it's full with local LGW-AMS traffic.
So LGW isn't an option on that market, any new runway at LGW would be for locos and holiday flights. Nothing wrong with that per se, but to get the biggest bang for the economy, it needs to be LHR runway 3. LGW can have another runway if it wants, I see no objection tbh, but I do not for a moment believe the rubbish from GIP about LGW being a hub. Ever.