Are there still Gat-Ex specific fares now that it has merged with Southern? Looking on the latest Advantix there doesn't seem to be any from Victoria - Gatwick.
That's right they were withdrawn.
But even when they existed they (obviously) only applied to Victoria-Gatwick. A ticket from anywhere else to anywhere else would be valid unless explicitly marked "Not Gatwick Express" but flows where such tickets exist are quite limited.
There would be absolutely nothing stopping someone with a ticket like Finsbury Park to Brighton, Peterborough to Gatwick, or anything like that, using Gatwick Express.
The assumption made by many people seems to be that only tickets routed "Gatwick Express only" were valid on the Gatwick Express, which was wrong. Now such tickets do not exist anyway. Any Permitted tickets can always be used on Gatwick Express for all flows where Victoria-Gatwick is on a permitted route.
NCoC states:-
NCoC said:
10. Tickets valid only in trains of particular Train Companies
The validity of a ticket may:
a) be restricted to; or
b) prohibit
travel in the trains of a particular Train Company or Train Companies. Any such restriction or prohibition will be shown on the ticket.
So if it does not say it's restricted to or prohibited from particular TOCs then they cannot stop you using that TOC. This is probably why Virgin can't (and, reportedly don't as I've never heard of anyone be chinged on one) enforce the supposed ban on their services with West Mids Day Rangers, for example.