Credit ratings don't have a currency denomination. Ratings agencies are only interested in currency exposure if it is a factor influencing a borrower's ability to service and repay debt.
With respect to rating "based on gold", the agencies might consider whether a borrower has cash flows that change with the price of gold. A fall in the price might have an impact on a producer's ability to service and repay debt. An unhedged producer would definitely have a lower credit rating than a hedged producer because of the uncertainty of future income.
