For peace of mind, allow at least 2 hours before short-haul flights when departing for Gatwick, and more for long-haul

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

For peace of mind, allow at least 2 hours before short-haul flights when departing for Gatwick, and more for long-haul. Add buffers for rush hour or weekend engineering.