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

https://augustrwra914.almoheet-travel.com/thameslink-to-gatwick-direct-routes-through-central-london-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.