Gatwick will never get a second runway. It just doesn't make sense spending anything expanding an airport, where given the choice, most of it's airlines would rather fly from to Heathrow.
If a third runway was built at Heathrow (the only realistic option), then Gatwick would become pretty empty. I suspect Easyjet would have no problem leaving Gatwick if there was space at Heathrow.
One runway and two terminals is ample for Gatwick.