Trains from the capital to the airport are operated by Govia Thameslink, which includes the Southern, Gatwick Express and Thameslink brands.
Only Southern is involved in the strike, but there will be knock-on effects for all services to the airport.
Southern services, which generally stop en route and are cheaper, will be badly disrupted, while the non-stop Gatwick Express is operating to an amended timetable.
Thameslink trains will also operate with amended timetables but are likely to be much more crowded and do not run from London Victoria.
The dispute between Southern and its staff centres on a plan to no longer make conductors responsible for operating train doors.
Unions say the practice of having drivers operate the doors is unsafe and that some passengers, like disabled travellers, will be put at risk.
They also fear there is a long-term plan to remove onboard staff from trains completely.
A Govia Thameslink spokesperson said: “Trains are calling at Gatwick airport, although the service is reduced”.
She added that passengers should check nationalrail.co.uk before travelling.
