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

https://telegra.ph/Gatwick-Airport-Shuttle-Services-Hotel-and-Terminal-Connections-08-21-4

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.