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

https://zionbjox380.almoheet-travel.com/gatwick-airport-public-transport-your-complete-options-guide

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.