This is correct, but that doesn't mean gold is a fiat currency. It's value doesn't come from decree. It is considered a hard currency.so again, the only value in gold is an imagined one.
Central banks are the world are hoarding gold right now. Iran is using gold to trade their oil with India, China, and Russia after the embargo was put into place.
No central governing authority decreed this to be. They have stopped using the current reserve currency USD and gold is the hard currency that has the most perceived store of value. Again not by decree, but because of its traditional perception as a reserve currency and store of value.