I can't find a ticket of code PBR from West Worthing to London Terminals (or if I search specifically for Victoria)
Can you find the fare on BRfares.com and link to it?
Edit: Ah you mean the Super Off Peak Day Return (
PDR); I will amend your post accordingly to avoid confusion.
Also just to add that brand restrictions are occasionally implemented by GTR, even though there is no provision for brand restrictions in the Ticketing Settlement Agreement (TSA). In this case the brand they are purporting to restrict you from taking is their Gatwick Express brand. This is unlawful in the opinion of many forum members and the matter will
go to court next year.
If you overpay as a result (either by purchasing a more expensive ticket, or if you are charged a penalty/excess/additional fare), you would be entitled to claim a refund of the difference if the court action is successful.