Gatwick seems to be badly served to Warsaw, Paris(&North of France) Norway, Jersey and Sweden by Easyjet and Ryanair. Might it be wise to consider some of those routes?
Gatwick is not that far away from either St Pancras or Ashford - from which there is a Eurostar with approx 800 seats on each train.
Norwegian have moved their operation from Stansted to Gatwick.
Flybe have a substantial operation from Gatwick and Southampton to Jersey and Guernsey
LGW to Sweden might be interesting - although Aer Lingus no longer operate from Ireland to Sweden so this would involve setting up a new station.