In short, you don’t get anything out of a gold standard that you didn’t
bring with you. If your government is a credible steward of the money
supply, you don’t need it; and if it isn’t, it won’t be able to stay on
it long anyway. (See Argentina’s dollar peg). Meanwhile, the
limitations on the government’s ability to respond to fiscal crises,
the necessity of defending against speculative attacks in times of
crises, and the possibility of independent changes in the relative
price of gold, make your economy more unstable. It’s a terrible idea,
which is why there are so few economists willing to raise their voices
in support of it.
Here is more, from Megan McArdle. I’ll add the related sentence "Who wants a pro-cyclical money supply?", and we don’t know what the new gold/dollar par should be, which means we risk a significant deflation during the transition to a commodity standard.
In the very long run, our monetary standard might be determined by what is least susceptible to counterfeiting or alchemy/nanotechnology. I doubt if this will help gold, and monetary economics will end up as a special case of a more general theory of encryption. One day they’ll solve Riemann’s Hypothesis and the price level will just go poof…!