Fine. BUT, if people keep getting caught out, then it suggests very strongly that this restriction is not being advertised clearly enough!
Let's have an analogy: say you have a step which a significant number of people keep tripping up on, and falling over. That indicates a usability defect in the design of the building, because the step is not obviously visible. The correct thing to do is to put a sign up saying "mind the step," or perhaps to put a yellow border or lighting on the step to make it more obvious, or to eliminate the step entirely. It is
not appropriate, or fair, to sigh deeply and say 'well, it's your fault you weren't looking where you were going" (and, in the case of the railways, also charge someone a hefty penalty for having been caught out.)
I reiterate my point that Gatwick being outside London is immaterial. It's about the same distance (30 miles) from Charing Cross as Amersham (27 miles), which is also outside London.
Yes, OP is responsible for checking, but surely the railways must bear
some responsibility for building up OP's expectations without clearly advertising the caveats. TfL in particular have an ad that says
Go Contactless, Go Anywhere. It has no mention
whatsoever that contactless payments are not valid beyond the Oyster zones!
Frankly, I cannot believe that no-one realises this is a problem. Honest customers are being penalised because they were conditioned to think their chosen payment method was valid. This is a problem. It needs to be fixed, preferably by
explicitly stating the limits of the Oyster and Contactless systems
at a point where the customer can still fix it by purchasing a valid paper ticket.
So, if OP is still listening, I would advise this: pay up, and then make a complaint in the strongest possible terms to the train operator that these restrictions
must be clearly advertised on the station information screens, and on the screens and signs on the train.