Gold is useful because it is consistent in quality, has minimal transaction costs today, you dont really "buy" gold only warehouse recipts, and the price of gold is equal everywhere in the world. Oil for example would not work because it varies in quality, has enormous shipping costs which vary from region to region etc etc.
That said gold is not money nor should money be backed by gold. Gold has no intrinsic value that gives it a right to determine a nations money supply. If an economy grows and the need for a larger money supply becomes apparent one would have to then find more gold - which is difficult but also more important the quantity of gold found is rather arbitrary. So for example during times of crisis like WW1 and WW2 nations moved away from the gold standard for those very reason - they needed more money and it was difficult to get to south africa and ship gold back - pesky u-boats. So money supply based on gold is really not that stable as you think or even desirable.
