So … like $12,000,000,000,000 of gold? I'm not sure if that's more or less than I'd expect.
It's definitely less than the world's total GDP, considering that China and the USA are both at or above that. So, it would not be possible for every country to have a gold-backed currency without a major decline in wealth. https://en.wikipedia.org/wiki/List_of_countries_by_GDP_(nomi... (Disclaimer, I am in no way an economist :) )
(I also wish we would stop thinking in terms of GDP, or that it's better to have a higher one)