It will be over a gradual 10 year period. Gatwick want secure a large airline for when the third runway at Heathrow goes ahead (Decision made but won't be publicised until some time after the elections). Gatwick will offer cheaper land for the HQ to be built on, as well as airport fee reductions. In return, Easy Jet will keep Gatwick as their main hub for years to come and will not operate and "major" operations from Heathrow.