If you are refused by the gateline staff, rather than arguing with them, you could also buy an adult rate ticket from the machine and travel with that, then submit a complaint to the operator of the gateline.
There is absolutely no guarantee that you would get your money back. However, you would have a compelling case based on the RST guidance. If you did this regularly, RST may not be thrilled about it if they found out. The complaint would be due to the train operator however, and not them. I can see no way in which multiple similar complaints to customer service could be considered travel irregularities or worse.
If you only travel occasionally from gated stations without open ticket desks it may be more fruitful to use this approach than waste your breath arguing with clueless staff.
I’ve seen a few cases at my station where the booking office has been closed (at a time when its scheduled to be open, as is increasingly common on GTR), and people have attempted to purchase PRIV tickets from the gateline staff, and been given a completely blank look (as it also quite common with many of the agency staff GTR use), and just been let through.
Could certainly see this causing some issues with revenue, though to be fair most GTR revenue seem pretty reasonable with all this.
More of a problem if this occurs at the end of the journey, though if one has made two attempts to pay and especially if facilities haven’t been available at times they’re supposed to have been, then I don’t think the TOC would have much of a leg to stand on.
I don’t think it’s reasonable to expect people to over-pay, and nor is it reasonable for people to try finding a ticket of equivalent value which is only going to cover part of the journey.