4) Other rail and tube options via central London:
This is one of several options, if using contactless cards and going via central London (zone 1) then all of them now cost the same. All these options cost £16.20 peak, £10.20 off peak. This cost is worked out automatically. Peak times are 06:30 to 09:29 and from 16:00 to 18:59 Monday to Friday, excluding public holidays. See
post 12 below for some technical details about peak/off peak timings.
The diagram above is not the quickest but it has the fewest interchanges: Take the Piccadilly line tube from either T5 or T3, get off at Kings Cross, follow the signs for the Thameslink services to Gatwick and Brighton, for direct services from there to Gatwick. However the transfer at Kings Cross is not easy, you need to go through to the far end of St. Pancras station and there are quite a number of long walk ways involved.
A better route, in my opinion, is to take the Piccadilly line from T5/T3 to Barons Court, then transfer over the other side of the same platform to the District line to Victoria. Then take any train other than the Gatwick Express to Gatwick. Works in a similar way for LGW-LHR travellers.
This strikes me as the best route to use between Heathrow and Gatwick so long as you have more than 2 hours 30 minutes until your next departure, it's reasonably fast, cost effective particularly off peak, can be done with contactless payment, and the transfers aren't too painful, the worst one being at Victoria station.
Another option is Piccadilly line to Green Park, transfer to the Victoria line down to Victoria station, then any train to Gatwick apart from the Gatwick Express. I don't recommend this since the connection between tube lines at Green Park isn't great, but it is technically quicker than changing elsewhere.
T3 (and T2) tube station