The way I see it is a Consortium of gold vault agreeing on a way to certificate and ensure that vault X contains Y grams of gold. After this certification process the vault owners can create Y tokens each one representing a gram of gold. These tokens will be minted on a Monero like Blockchain. Moving a token from on wallet to another will be completely private. A fraction of a token can also be moved from wallet to wallet, giving high granularity. Weather underlying these transactions will be a Monero token used as "gas", is open to debate. It makes things complicated but it can give more flexibility. This gives you what you want. Grams of gold will be used for transactions and not Dollars.