Gold is a chemical element valued by Humans. The element is soft compared to silver, another element of great vaule.
Gold is used in the making of gold-pressed latinum, to suspend the liquid latinum within it. Gold can also be melted into arrow heads, coins, and statues.
The Animus, an Old One, could control beings that were in contact with gold, which is how it came to control the Zarbi. Gold is also a major weakness of the Cybermen. Because of its non-corrosive nature, when gold enters their respiritory system, it chokes them to death.
Gold lost its value by the 24th Century because people had found a way to replicate it.