There are many benefits that come with diversifying a retirement account with gold or Bitcoins (or other cryptocurrencies).
Gold is an extremely reliable and solid way to store money, since it's a way to safeguard possessions by protecting one's buying power/conditions (gold is not affected by devaluation the way regular currencies are).
Bitcoins and other cryptos may also seem as an alternative to regular retirement fund options or even gold, but it ain't (at least for now). Even though cryptocurrencies are still very unstable (monetarily speaking), they are (or were according to some experts) a way to store and easily transfer money anywhere. They are very promising in terms or transfers, safety measures (crypto-exchanges and users can get hacked, but the cryptos themselves operate in a relatively safe way), and even though it may be a way to earn some extra money (through trading/speculation), they are not comparable to gold when it comes to monetary safety (stability); that's why they might be a good investment option but the investor must be extra careful about putting a large portion of their retirement fund into them.
In other words, Gold is mainly reliable and stable (a way to protect you buying power and worry less about inflation/devaluation), and Bitcoins are an opportunity to make more money and a reliable and cheap way to transfer money anywhere
