I used a BZ6 ticket on Gatwick Express and was told by the Southern clerk it wasn't valid, but the gateline dragon at the GatEx platforms at Vic it was. The platform despatch at Gatwick said it wasn't, the gateline dragon at Gatwick said it was... :P
Southern give some of their staff extremely poor training, and especially so in the case of their staff who have anything to do with their Gatwick Express route trains. However it is very rare that Southern bother with on-train checks on either their Brighton Express or Gatwick Express routes. So most of the time you're dealing with gateline staff.
It is rare for gateline staff of any TOC to be adequately trained in ticketing matters, but it is especially so of Southern.
There are some Southern gateline who are
very knowledgeable, but they tend to have acquired their knowledge on this forum, and NOT by the training (or lack of) provided
One of whom knows exactly what it's like to have a valid ticket rejected because of non-existent rules that were a fiction of the gateline assistant's imagination (before he started working for them).
So, yes, expect to be told valid tickets are invalid. But that does not in any way invalidate the tickets so you can just ignore them, providing you're absolutely confident it's valid.