From my info you would be also paying too much according to my sources:
Anytime Singles
Gatwick to Boundary Zone 6 - £7.30.
Gatwick To East Croydon (Zone 5 which every London-bound train goes through and all non express should top at) - £5.30
I did take a photo and post on a forum (here or elsewhere) a few years back when it happened. The extension from Z6 to Gatwick was cheaper GOING to Gatwick than the ticket I got back from Gatwick to Coulsdon South (I think). My Z1-6 was a season, so the train didn't need to stop there.
I got a Gold Card discount on both, and I'm sure it was about £1 more coming back. Presumably a Gatwick to BZ6 should have been the same as in reverse, or else it's very confusing!
I could happily pay the extra and claim later, but at Hatfield when I am going to Heathrow airport the delay with the arguing that a ticket doesn't exist means that;
a) I could miss the train I planned to get (last time, I got the ticket in advance to prevent this being an issue)
b) If they won't sell me the ticket at all (as it supposedly doesn't exist), I must go to Paddington on my existing season and queue up there (they know how to do it) and because of the time the train arrives at King's Cross and the time to get to Paddington, queuing later likely means me missing one train and having to wait 30 minutes for the next one... so that member of staff means having to go 30 minutes earlier, or be in a rush at the airport!
In either case, complaining later doesn't really do much and there are lots of members of staff there, including agency staff, so I'm not sure how effective any briefing from FCC would be.
The last time when I did get the woman to sell me a ticket, she said it stated 'NOT LONDON' and I couldn't use it as I was going in to London and taking the train from there. I don't think she understood when I pointed out that the Not London was only on the bit from Hayes & Harlington to Heathrow, which wasn't going via London. Maybe both of us misunderstood it, but the grippers happily accept them on the train.