The use of the term digital gold is done to describe some of the attributes of cryptocurrency to that of gold. This is very common with those that follow the Proof-of-Work protocol since it is minded.
Bitcoin is often termed digital gold.
The reasoning stems from the fact that the currency has a capped amount that can ever be mined. This provides it with scarcity, which proponents believe will make it a digital store of value, just like gold has been used in this manner in the physical world.
From this, many envision a day when corporations and central banks will hold Bitcoin on their balance sheets as part of their reserve. There is also the claim that this will end up being used as collateral against loans.
Bitcoin experiences a high degree of volatility which makes it difficult to see its store of value features along with the ability to act as collateral, especially within short-term lending.
Much of this could be due to the relatively market capitalization. It runs about 1/10th that of gold and less than half of silver. If the market cap grows, volatility could diminish, solving the two issues just mentioned. One thing to keep in mind is that scarcity, due to the cap on the output, will always carry with it a degree of volatility.
Since the underlying network is a decentralized blockchain, there is full transparency as to how much Bitcoin is available. However, there is no way to tell how much is lost due to private keys being misplaced or locked into hard drives that are not accessible.