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

https://telegra.ph/Southern-Railway-to-Gatwick-Affordable-Alternatives-Explained-08-21

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.