A gold coin is an item in Evony TKR (The King's Return).
Gold Coin is one of the collection items in the exhibition for the Art Hall. It is one of four parts in the Coin Hall. After the set is completed at three stars, you can trade additional Gold Coin fragments for gold.
The description of Gold Coin in Evony TKR says:
Currencies with collection values. The important exhibits in Ancient Coin Hall.
Gold Coin is known to potentially be obtainable from opening Artwork Fragment Chest I, Artwork Fragment Chest II and Artwork Fragment Chest IV. The links for each item are below:
Note: This list may be incomplete.