Digital gold is said to be an attribute of Bitcoin because in the year 2140 with 21 million coins generated ("mined"), no more bitcoins will come into existence. The digital gold analogy is misleading. Gold has always been used as a hedge against inflation because it has intrinsic value, and Bitcoin is thought to be a hedge against inflation because of its 21-million cap. However, every year, new gold is being mined out of the ground, whereas newly generated bitcoins will eventually cease. Nevertheless, what the world will look like in more than a century from now is anyone's guess. See
Bitcoin.