If you are using Oyster to pay as you go for this journey, you need to leave the train at East Croydon and touch out, and wait for the next train to continue your journey (as there are no Oyster card readers on the platforms there).

It is sometimes cheaper (rather than lazy) to issue from a named station rather than Boundary Zone X, and this is only a disadvantage if the Travelcard held is a Day Travelcard, in which case the combination isn't valid.

Boundary Zone X tickets are available as a wide range of ticket types (not just day returns) depending on the origin and destination in question. I don't have access to the fares database at the moment but will endeavour to look up the various options for this journey when I get a chance.

Finally, if the OP's Travelcard is an annual one, he's entitled to a 34% discount on all tickets in the "Network South-East" area that aren't covered by his Travelcard, except not Gatwick Express.