The CIS should interleave between various screens. Did you watch the screen to see if the information changed?
I stayed there for some time, and it wasn't even alternating between page one and page two of the engineering works notice, simply showing the first page which cuts off mid-sentence. This is a problem that no-one at Thameslink seems to have taken the care to notice.
The same goes for the identical screen on the escalator landing on Platform A.
I notice you missed the map on the wall on the right-hand side of the concourse behind the barrier line?
I was actively looking for any clue to Oyster validity and didn't see the one behind the barrier line. That implies that it is not obvious enough.
And I come back to my point about the map itself: one has to really,
properly scrutinise it to see that Oyster is not valid to Gatwick. (And if we are being pedantic, no mention at all is made about Contactless cards.) Unfortunately I didn't get a photo of it, but the text indicating that "Oyster can be used in the shaded area" is tiny and not somewhere immediately obvious.
And the manned ticket office?
And the manned information desk on the left of the barrier line?
And the staff manning the barrier line?
And the staff around on the concourse?
Any of those could have provided the information you needed.
You need to actively seek out this information (which will be no good if it's at night and the ticket office is closed and the barrier is open, oras will happen next weekif the ticket office is shut completely for refurbishment.)
If you have no reason to believe that Oyster/Contactless would
not be valid to Gatwick (as the OP did) there is no reason you would seek out this information whatsoever.
I would also point to Thameslink's own pages about
Oyster and
Contactless, all of which use the wooly statement that it is valid for "getting around London." Neither page has any mention of the boundaries. On their
page about Gatwick, there is not a shred of evidence to suggest that Oyster or Contactless wouldn't be valid. I have to click through to "more information about Gatwick", then expand the Ticket Office section, before I finally find the information I want:
To continue my experiment, I also took a train from St. Pancras to East Croydon (then continuing via Gatwick to Three Bridges.) There was, to Thameslink's credit, a single PIS announcement on the approach to East Croydon that Oyster was not valid beyond the next station. But by my reckoning it lasted ten seconds and was not repeated, so if you weren't paying attention, or couldn't hear it due to a PA fault or ambient noise, it would have been very easy to miss it.
As
DaveNewcastle says, if in doubt, ask. But, when plenty of people
repeatedly turn up at GTW in no doubt (mistakenly) that they can use their Oyster or contactless, clearly the information that Oyster/CPC is only valid as far as Coulsdon South is not advertised clearly enough.
Hence my (repeated) advice to the OP: pay up, and then complain strongly that the fact Gatwick is outside the boundary is
simply not clear enough to passengers unfamiliar with the system.