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

https://spencersgwz465.cavandoragh.org/gatwick-airport-public-transport-your-complete-options-guide-4

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.