I recommend the Return, given that that there could be quite a queue for the Gatwick ticket office and ticket machines when you return there from wherever you are going. There certainly was last time I flew into Gatwick, with many of the intending passengers unfamiliar with what they needed to buy and how to buy it.
Agreed.
It is possible to potentially save a little money by using Oyster or contactless to pay for the journey between East Croydon and Gatwick, but to do so one has to leave the train at East Croydon, exit the ticket gates (in the OP's case, using their Freedom Pass) then re-enter using an Oyster or contactless card, then exit at Gatwick using the same Oyster or contactless card - and do the same on a northbound journey.
The Oyster/contactless fare is £3.10 off-peak, or £5.30 peak times (for journeys starting Monday-Friday between 0630-0930 and 1600-1900 - in terms of travelling from East Croydon to Gatwick, that would be when you touch in using Oyster/contactless at East Croydon).
Quite possibly too much of a faff for many though!