It is standardized as paper currency, but no more. So if i would have to pay exactly 1.67€ for some service, then i need a 1€ bill a 50 eurocent coin , a 10 eurocent coin, a 5 eurocent coin, and 2x 1 eurocent coin.
So that is 6 different units that i need to pay that specific price (now with the current rate of inflation obviously pennys dont matter anymore, but lets just assume they do).
The same with gold, unless you got all sub-coins in gold at you in your pocker, you need to cut the coin in half in order to pay in a specific gram.
For example if you got 2 gram coins, but you have to pay 1.65 grams of gold, then what do you do?
Coins usually come in different denominations, just enough to settle in cash the majority of accounts you may ever think of. In medieval history gold had been used to make large purchases, silver was used as change. Since we are talking not about past but about future (and post-apocalyptic one at that), it is pretty clear that 7.62×39mm (aka AK-47) ammunition will be used as a small coin...