RPSI is a program that essentially stamps specific breeds of horses as worthy (or approved) for breeding. If your RPSI approved mare was bred to an RPSI approved stallion, the resulting foal would be eligible for a brand and set of papers.
During their testing, they award the foals with "Gold" "Silver" or "Bronze" - Gold obviously being the best. Said horse must score within two certain points, thus be awarded with G/S/B status.
That basically sums it up, but I would suggest contact the RPSI :)