It's the perfect recipe for failure.
The whole point of crypto is that you verify, and not trust. How can you verify that Brinks actually has enough Gold in its vaults to back up whatever Gold backed crypto? I don't want to trust on an "independent" entity doing the verification part and they then release a report that says they are backed. If I can't see that they are backed myself, their words hold no value.
Bitcoin is Bitcoin. You can verify everything easily and fast with how transparent this whole system is. Why go back to something we try to get rid of?
The most important is that with cryptocurrencies if you send them to your own cold or hardware wallet it belongs to you unless it is stolen). With buying/trading precious metals in many countries (unless you get it with much worse spread) you just get a confirmation that you are an owner of some ounces of eg. gold in a gold bar stored 1500 miles away from you.
When really needed you will have no access to such gold or it will be captured by authorities (as it already happened in the USA in 1933 with Executive Order 6102 by F.D.Roosevelt).