The expansion of Pier 6 is more about achieving a 95% target of gate handled flights and eliminating more bussing as that's what the airlines at Gatwick have requested it's not necessarily about increasing capacity or passenger numbers at the airport as that is limited by the single runway and having the emergency runway in use with the current main runway is more or less a non starter.
All that information regarding expansion of Pier 6 can be viewed on the Gatwick Capital Investment Page on their official website.