Check your terminal before departing; Terminals 2 and 3 share a station

https://www.scribd.com/document/1074920514/Getting-to-Heathrow-from-Gatwick-Easiest-Inter-Airport-Transfers-178501

Check your terminal before departing; Terminals 2 and 3 share a station, while Terminals 4 and 5 have their own dedicated rail stops on the tube and Elizabeth line.