Easyjet have approx 44 % of summer 2015 ATMs at Gatwick. Add Monarch, Thomas Cook and Thomson, and you hit almost 57% of ATMs. Gatwick is not exactly known for being empty at peak times. Granted a 2nd runway would mean many more ATMs become possible, but this is still a sizeable number of slots
The moment Easyjet start offering connecting flights (ie airline bears risk of delays and luggage can be checked through) is the moment Gatwick might have a non-zero chance of becoming a hub. Until then, Gatwick hasn't got a hope of becoming a hub