I bought an "FCC Only" Gatwick Airport to London Zones 1-6 to travel to Clapham Junction. The only reason I bought this is that the so-called fast ticket machines at Gatwick have such non-working touch screens that I couldn't coax it into selling me a simple single ticket to Clapham Junction, which would have been cheaper.
But when I arrived at the exit gate at CLJ, it wouldn't let me out. This must be one of the 'non-helpful' ticket gates. Unfortunately there were no staff around so I had to use my Oyster card (incurring an 'incomplete journey' extra fare) to get out of the station.
It's very hard to find out whether "FCC Only" is supposed to be valid to CLJ but it does clearly say "London zones 1-6" (Clapham Junction is in Zone 2). As it happens, the first train out from Gatwick was an FCC (I think ...) train to East Croydon, where I changed to a Southern train to Clapham Junction. I can't for the life of me see why this should not be allowed. Perhaps if I had got out of the station at East Croydon, then gone back into the station using the same ticket, all would have been allowed??
To be honest, the ticketing system at Gatwick Airport seems to have been designed to discourage people from using anything other than the Gatwick "Express" (which on that particular day and time was cancelled ... again). Now that GatEx doesn't allow you to buy tickets on board, there are gates at the airport station, but, once through the barriers, there are no platform indicators, apart from the "most popular" destinations - who decides what is popular?