Hi Thomas,
Was this a contactless card?
There probably isn’t any refund due because extension fares to Gatwick are not just simply the zones covered. You may remember the article Gatwick The Truth on here a few years back. Because of the bonkers cheap fare from East Croydon to Gatwick the system never charged the full London to Gatwick return fare. DfT/GTR whinged and thus the CPC extension being more than the Oyster extension concept was born.