Gatwick has North and South Terminals connected by a free shuttle

https://www.scribd.com/document/1077066259/Heathrow-to-Gatwick-Airport-Transfers-Coach-Taxi-or-Train-174457

Gatwick has North and South Terminals connected by a free shuttle. Trains arrive at South Terminal; allow 5–10 extra minutes to transfer if your flight departs from North.