Bullion is usually not the basis for a credit economy. I remember reading Graeber's "Debt: The First 5000" about this point, but I forget the details. And if we're talking about an "illusory basis" (i.e. much more money than there is bullion) - so in a sense it doesn't matter all that much whether it's Gold or BitCoin. You won't have the distributed nature of BitCoin then, because banks would create "BitCoin-backed credit", i.e. banks create the money.