BritRail Pass from Gatwick to London

A quick google search located the terms and conditions pretty easily. The answer is in the 12th paragraph, called General Conditions, where the Gatwick Express, Heathrow Express and Stansted Express are discussed.

If you are using East Coast from Kings Cross you can use First Capital Connect from Gatwick to Kings Cross direct needing no Underground. If you use services to Victoria, Southern or Gatwick Express, you need to use the Underground, a bus, or a taxi to get to either Kings Cross or Euston, (depending on your route to Edinburgh).

Thanks for the replies. I found my answer.