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

https://miloanod546.wpsuo.com/gatwick-airport-travel-times-what-to-expect-from-different-routes-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.