You should be able to ask for an "extension from Zone 6 to Gatwick Airport", and present your Oyster card - these are only available as day returns. Your ticket will then read "From: Boundary Zone 6", although some ticket agents are lazy and will put in the name of the boundary station instead (strictly speaking, this isn't the proper way of doing it, and some train companies get funny about it now).

Oyster Extension Permits no longer exist (and were only for journeys within the Oyster Zones anyway).