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

https://finnbqtp289.tearosediner.net/brighton-to-gatwick-airport-train-times-fares-and-tips

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.