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

https://simoniefh394.image-perth.org/heathrow-to-gatwick-airport-transfers-coach-taxi-or-train-2

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.