Ah - I've not had the pleasure of Croydon for a while. The ELL goes to
West Croydon though - is that near East Croydon?

Sorry, I should have said "at the price of an *extra* change..." as I
had in mind you changing at New Cross Gate and then again at E Croydon.
But if you buy your ticket from there to Gatwick in advance online and
collect it before you travel I think you only need touch out with your
Oyster and then board the next train. I can't say if it is practicable
to aim to reboard the *same* train as I've only used that route with a
Freedom Pass

But wait a minute -
I've just looked at the route map and the stop before - Norwood
Junction - is shown as an interchange to Gatwick. But that's £8.20.
Big drop if I can get to EC - odd that the fare drops so much in one
stop.
The offpeak Oyster fare to Croydon is actually £2.90.

Yep for West Croydon. East is£2.80.

--
Robin
reply to address is (meant to be) valid