It makes total sense from the perspective of easyJet to oppose 2nd runway @ LGW in favour of 3rd @ LHR.
They maintain their scale at LGW, and less room for an absolute invasion by other LOCO's at the airport. LGW will remain london airport #2, so it will suit easyJet to remain #1 there. An additional runway could change all that as it could provide as serious opportunity for FR DY VY etc to expand there.
An additional expansion of runway capacity @ LHR would give ezy a golden opportunity to fly head to head against much higher cost rivals. Let's call it a honey pot , one where FR will be unwilling to go.
So from an Easyjet perspective it's all about the relative cost position. Expand LHR and challenge higher cost rivals, and disable LGW where rivals tend to be lower cost eg DY, FR, VY etc.