I have a Travelcard for TFL Zones 1-6 and want to go meet my girlfriend when she arrives at Gatwick this weekend. I don't want to pay for a ticket from Central London to Gatwick since I can virtually travel most of the distance there for free.

Can I use my Oyster Card + Extension Permit to get myself there? If not, is there another option that takes advantage of my Travel Card and don't have to pay from Central London?

Cheers,
-Matt