Not convinced that LGW would suffer that much, yes of course the mainly long haul routes of carriers that don't have access to LHR now would be ripe for migration and some short haul routes that are business centric might move, but for many LGW is very accessible and the access cost to airlines will likely remain below that of an expanded LHR.
Easy are very unlikely to move their entire operation to LHR but a base for their business routes would be logical