Due to engineering work at Stoats Nest Junction, there will be no trains between East Croydon and Gatwick Airport. There will be extra fast trains to East Grinstead, where buses will run to Gatwick and Three Bridges, and from 27 Dec to 1 Jan there will be trains from Gatwick to Victoria via Three Bridges, Horsham and Dorking.
http://www.southernrailway.com/your-journey/plan-your-journey/christmas-train-services/
http://www.southernrailway.com/your-journey/plan-your-journey/christmas-train-services/