Looks like what happened is, the guard switched roles and denied he could buy on board as it's only GatEx who allow that and the train became Southern (but it is advertised as GatEx - which doesn't even exist as a company*) and either he refused to pay full fare and was later PF'd when an RPI boarded, or an RPI got there first, doesn't really matter the exact circumstances.
Meanwhile if you get one of these trains between Gatwick & Victoria, the SN employed guard may DENY that he is employed by SN and may DENY that the train is operated by SN (even though their website admits this) and refuse to accept SN only tickets. I would love to see someone say "see you in court". GatEx can't take you to court, so Southern would be arguing in court along the lines of "The accused took a train operated by us, on a ticket only valid on our trains" haha I would lose all faith in our legal system if they won that!
Last week a member of this forum encountered a lying Southern guard (in disguise!), but sadly didn't want to stand his ground.
They want to have their cake and eat it.
They want to disguise themselves as a DEFUNCT company between Victoria and Gatwick, to deny cheap fares. They then deny that they are GatEx when it doesn't suit, south of Gatwick, despite the trains being (falsely) advertised as such!
They can't have it both ways!
If you travel on Gat Ex on a Southern only ticket, and (rightly) refuse to pay, Southern will have to argue in court that you are invalid, and Southern will have to admit that the train is operated by themselves (their website says they do!) and not a defunct company, so I really would like to see someone defeat them on that issue too!
* Gatwick Express Ltd still exists but it has the registered office of National Express, Birmingham, so its not the same company