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

https://troyuiov591.huicopper.com/stansted-to-gatwick-airport-how-to-transfer-between-airports

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.