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

https://lanekphi755.almoheet-travel.com/gatwick-airport-terminal-transfer-shuttle-and-walking-routes

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.