The value of gold has not changed throughout history, what changes is its price, its price.
The reason is that the price of gold is set in dollars, which is a fiduciary currency, in markets dominated by a small number of banking giants.
Fiduciary currencies (the usual ones of legal tender) are created at will, without material cost: it costs the same to create 1 million than 1,000 million, it is an order on a computer keyboard.
The value of the fiduciary currencies is disconnected from the reality of the economy, it is totally apparent.
The price allows to compare the value of goods for trade. Thus we can evaluate if a good is expensive or cheap, comparing with other goods that can be purchased for the same price.
The price is fixed mainly in London and in New York, in dollars. They are mainly financial markets, that is, only 2% of transactions involve the physical delivery of gold.