If you’re transferring from Heathrow, use the RailAir coach or Tube to Victoria, then train to Gatwick

https://simoniefh394.image-perth.org/heathrow-to-gatwick-airport-transfers-coach-taxi-or-train-2

If you’re transferring from Heathrow, use the RailAir coach or Tube to Victoria, then train to Gatwick. Direct coaches also run between airports at certain times—book ahead.