The ticket buying chaos at the railway station is compounded by the absurdly complicated fare structure for fares to London.
Oyster sounds like a step forward but....
(1) While it has revolutionised travel in London, the oyster technology is near the limit of what it can do. There are only so many price buckets it can cope with and its now close to that limit. Note that tfl aren't pushing development of oyster now, they're pushing contactless bank cards, and already 20% of London journeys are now on contactless bank card.
(2) Although there is now only one train company operating ALL trains between Gatwick and London (legal name GoVia Thameslink Railway Ltd) it is persisting with separate brands (Thameslink Southern Express), and persisting with separate price points for the different brands. The legality of that is hotly disputed, but put that aside. In terms of oyster, oyster can only have a single fare for a straightforward "point A to point B" journey, so oyster can't be introduced until this ticket chaos is sorted out.