Hi Simon,
The 60+ Oyster card and the Freedom Pass are both off-peak season tickets so there is no penalty for not touching in or out within the zones.
Usually you would buy a ticket from boundary zone 6 to your destination, especially if there is more than one route (eg to Epsom). In the case of Gatwick Airport you are best to buy Purley to Gatwick Airport tickets. Off-peak they are slightly cheaper than the BZ fare and Purley ensures you are valid via either Redhill or the Quarry route. The train doesn’t need to call at Purley because your 60+ is a season ticket.