>>9Legal currency has a defined value and must be accepted by vendors/banks/organizations of a country. You're probably referring to gold standards, which ties the currency to specific amount of gold. Gold itself is tied to its value as jewelry/dentistry material and use in electronics.
Gold in abstract sense has no objective inherent value(unlike water which is required for life and inherently valuable), just rarity and permanence(doesn't rust) - people deciding on it as store of value and things like gold teeth make it valuable in specific contexts: if people decided a specific stone is a better store of value, gold would not hold that position of "reserve currency".