@Wolfie. GX tickets are permitted on SN services always. But not V/V. The exceptions being on the peak trains which start/ terminate beyond Gatwick. Mainly where passegners board/ alight as (eg) Haywards Heath on a Gatwick Express service.
That may be the official Southern line. There have been MANY threads on here over the validity of that approach (and the fact that Southern ALWAYS back down!) - I suggest you do a little searching....
Advance tickets, such as Vic-BTN for £6 do not permit a break-in-journey and therefore if they tried to leave at Gatwick, as it cheap, they will be fined. AS that advance ticket is for that booked train (if applicable) to that destination.
I fully agree with everything you say.... I also note the pooh-storm that engulfed East Coast and SWT when they tried to apply such restrictions on ending short rigidly (again both the subject of past threads on here) and can guess exactly what would happen to GoVia Thammeslink if they try the same....