Thanks for that. However, I’m afraid the following newer thread just made your suggestion a little bit riskier... though in that case it happened in the ticket office managed by SN rather than (possibly) GX.If you ask to buy a return to Gatwick from the dedicated Gatwick Express ticket office at Victoria, and show them a Travelcard, they will give you a BZ6 - Gatwick return without being prompted.
https://www.railforums.co.uk/threads/ticket-office-at-victoria-refuses-to-sell-me-a-ticket.182897/