Soon it becomes very clear that gold was never "declared" to be a form of money by any "authorities" but rather, became money due to natural market forces.
Interestingly enough, this is actually false - gold coins were originally used by kings as a way of forcing their citizens to support local armies by making it the only legal tender for taxation. Genuine decentralized societies tended to use proto-Ripple-style credit systems. Read David Grabeber's
Debt: The First 5000 Years.