From Croydon, hop on Southern or Thameslink from East Croydon station

https://knoxsmdj610.cavandoragh.org/heathrow-to-gatwick-airport-transfers-coach-taxi-or-train-2

From Croydon, hop on Southern or Thameslink from East Croydon station. Trains to Gatwick are frequent and fast, often taking less than 20 minutes platform to platform.