Gold is money, because of its fundamental nature.
Gold is the perfect commodity for exchange for the following reasons:
Gold is liquid and easily traded, with a narrow spread between the prices to buy and sell (about 1%).
Gold is easily transportable, because it has a high value for its weight.
Gold is money because it is divisible, you can divide it into coins, or re-melt it into bars, without destroying it.
Also, gold is interchangeable. It can be substituted for another piece of gold with no hassle.
Gold is also nearly impossible to counterfeit, as genuine gold is easily recognizable.
When measured by weight, gold is easily countable, and verifiable.
Gold is money because it is a great store of value. It is not subject to decay, rot, or rust.
Gold has an intrinsic value, because it is rare, highly desired by the world over, and is a luxury item.
There is not a single other commodity with those attributes, except, perhaps, for silver. Since gold is too valuable to be used for small transactions, there is potentially more monetary demand for silver. When gold becomes money again, silver will be desperately needed to make change. Platinum and palladium may come close to gold, but they are not so easily recognized by the masses, and are used mostly by industry.