While it is true that directly handing someone a piece of gold as payment has Zero transaction cost, in the real world the trade of gold is still quite expensive. Due to its weight there is an intrinsic transport cost even at the local level, sending physical gold overseas especially large amounts will incur significant cost. Most gold that is transacted is done through specialist companies that also have insurance overheads. While there are similarities between gold and bitcoin, when it comes to real world use, digital assets are far more efficient, thus cheaper to transact with.