No, you don't - I think there must be some confusion here. It is a one cabin upgrade, subject to availability (of revenue fare classes if booked through a Travel Agent, of redemption availability if used via BA). There's a whole thread dedicated to them linked in the Dashboard.

The only time you'd have to pay extra is if government taxes increased between cabins, for example upgrading from Y to Y+. This is a fairly rare use of a GUF - most would be from Y+ to J or J to F.

However, as you believe you'd only just scrape Gold, these are probably currently out of your reach anyway.