Gold have been around for decades a d we now have Bitcoin, that act as alternative to gold in terms of investment, let say that Bitcoin is a dime of digital gold since Bitcoin have the components of both gold and digital currency as a feature when you take Bitcoin as a digital assets, gold long term trust have build the market capitalization we are seeing today, and as much as Bitcoin keep the current trend it will definitely surpassed Gold in market cap in near future.
Bitcoin was never an alternative for gold, it was simply a new investment option alongside other investments like gold, oil, stocks, commodities. Because they are different, gold is a physical asset while bitcoin is just a virtual asset.
As someone pointed out and if you take the time to learn, you will also know that physical gold makes up the majority of gold's supply and capitalization. How can a virtual asset replace the role of a physical asset? So bitcoin surpassing gold is very unlikely.