A freedom pass isn't accepted on the Gatwick Depress. Sorry mean Express.
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You could always get a Thameslink service from London Bridge or East Croydon.
Disagree - the Gatwick Express is treated as a Southern service for ticketing purposes when the journey being taken is beyond Gatwick. (This is in addition to the whole issue of Southern/GX being the same franchise, so all tickets are technically interavailable between them anyway )