For peace of mind, allow at least 2 hours before short-haul flights when departing for Gatwick, and more for long-haul
https://andreskqov525.theburnward.com/gatwick-airport-terminal-transfer-shuttle-and-walking-routes
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.