I will be arriving at Gatwick on a Saturday and then have to go to Earls Court. I will purchase a London Travelcard but I am unsure if I can use it on the entire journey from LGW. Some sites say it's valid on National Rail services, some say exceptions apply and it's only 'within London'. I know I won't be able to use it for the Gatwick Express but if I can use it for a regular train and don't have to purchase a separate ticket, that'd be nice.
Furthermore, I am confused with the Zones system. I read since January 2014 the Travelcard is zone 1-6 only however the local Visit Britain Office here still sells Zone 1-2/4. What gives?
And last, I depart on Monday at 13:00 from LGW. At what time should I leave from Earls Court to arrive around 11:00-11:30? What tickets would you recommend for that day. I might buy another off-peak travelcard (8 GBP), if it's valid to Gatwick, or should I buy a single Tube ticket and another separate trainticket?

Hope you are able to give me some advice on the transport in London.
Thanks in advance!