The staff at East Croydon were wrong; it's permitted to travel via East Croydon on a South Croydon to Gatwick ticket. You can also break your journey (i.e. get on or off) at East Croydon if you like.
That being said, you could be accused of having committed an offence under Railway Byelaw 9(2) by letting yourself through the barriers. It's fairly unlikely you'd be prosecuted for this, and as far as I can tell, you can't buy e-tickets between South Croydon and Gatwick so you couldn't be identified through that. But if the staff cared sufficiently or the British Transport Police were involved, I suppose it's possible they might look at the CCTV footage to try and identify you in case you pass through East Croydon again in the future.
If something like this happens to you again in the future, I'd suggest cooperating with the staff and providing your name and address if they ask for it. Let the police attend if need be. That way, by having complied with their instructions and having used a ticket in an entirely valid manner, you have done nothing wrong and can't be accused of anything.
Normally, I'd suggest raising a complaint with the train company in an instance like this, but given the circumstances, that is perhaps not a good idea.
