Without a return itinerary he’d have to go the valid routing.
If the OP didn't happen to know that the route on the outbound itinerary goes outside the Gold Card area, then I don't think it could reasonably be said that he ought to have known when an accredited retailer was telling him otherwise.
And if that's correct (which I acknowledge is debatable) then I really don't see how he could magically be expected to know the same routing would be invalid on the way back.
I do wonder if some people interpret the right to travel in accordance with the itinerary against which the ticket was sold without having the reason for it in mind. The reason that right has to exist (despite not being stated in the routing guide afaik) is because it would be unreasonable to expect the consumer to know or suspect that they'd been sold something that wasn't valid. In this case, it is either reasonable to expect the OP to know he couldn't go via Bristol on that ticket or it isn't. I really don't see how one can accept he couldn't reasonably know when he makes the outbound journey but then expect him to check the route against the Gold Card area map before returning, when the return route is the same. Not having a return itinerary may well carry an obligation to check things like peak/off-peak or TOC restrictions, or time restrictions on the use of the Railcard, but in the reasonable customer's mind the validity of that ticket via Bristol has already been established by the fact it was sold to him on an itinerary via Bristol. It's not as if we're talking about two entirely separate journeys or different tickets, here. So in that respect, I do wonder if the right to do things that would otherwise not be valid based on an itinerary is interpreted too narrowly by some?
Conversely if, as a matter of fact, the customer does know that an error has been made by the retailer that means their ticket is invalid and there is evidence of this, I would not personally even think about relying on an itinerary which I know to be erroneous and can be shown to have known was erroneous before I travelled. That is just asking for trouble.
There may, of course, be cases where the customer doesn't know about the retailer's error when they make the outbound journey but does know before they make the return journey (e.g. if they are alerted to the issue during their outbound journey in the course of a ticket check). That is a situation I am less sure about. The customer could not, in my view, then return via Bristol on that ticket, but if it were me I would try to get the retailer to issue me with a new ticket, valid to make the return journey via Bristol, for no additional charge given they sold me a return ticket.