You can't use a Travelcard to Gerrards Cross because it's outside the London zonal area. The last station, on that line, within Zone 6, is West Ruislip, at the top left-hand corner of this map:
http://www.tfl.gov.uk/assets/downloads/lon_con .pdf
The single fare for the journey is �10.50. A return ticket costs �19.20 unless you travel after 0930 (on a weekday) or any time at weekends, when you can get Saver Return for �18.20 or a day Cheap Day Return for �10.70.
A 6-zone Travelcard, to get you to West Ruislip costs �13.20 for travel before 0930, Mondays to Fridays. At other times it costs �6.70
A single from West Ruislip to Gerrards Cross costs �2.80 and a Cheap Day Return (which doesn't appear to have any time restrictions) is �4.40. You could buy your ticket, for this part of the journey, at Wimbledon; you wouldn't have to alight at West Ruislip.
Chris