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

https://stephenjvpn176.theglensecret.com/thameslink-to-gatwick-direct-routes-through-central-london

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.