Other metals used to create fake gold coins can not match a genuine Gold Coin in size and weight. For example, Gold is heavier than lead by 30%. A coin made of lead would have to be bigger in size by about 1/3 to weigh what a gold coin weighs, so a lead coin would not pass either the size or the weight test.
Other cheap metals such as zinc, copper or steel are even lighter than lead. There are only two metals that come close to gold in weight. Platinum, which costs more than gold, and Tungsten, which is so brittle it can't be stamped into a coin.