The risk-free rate of a x-currency denominated instrument can be determined from treasury bills, interbank borrowing rates (e.g. LIBOR), overnight index rates or interest rate swaps.
What instrument may be indicative for a risk-free rate of an instrument, which is denominated in a precious metal, specifically in gold? That is, the counterparties to such an instrument pay and receive payments directly in gold. The risk-free rate for gold is primarily needed for discounting reasons to discount an amount of gold from the future to the present date. For instance, 1 XAU (i.e. 1 ounce) in 3 months is actually 0.9998 XAU today for a positive discount rate. Something like that.