I read some time ago (unfortunately, I've not preserved the link, but you can find similar opinions like this Bloomberg article) that gold would be worth only a fraction of its current price if it wasn't for Central Banks hoarding it. If Central Banks begun to sell a significant part of their gold, the price would plummet, because the demand for industrial use would not be enough to sustain anything near the current price.
I think gold is mostly valued by its tradition as "something valuable", which goes back to Antiquity, and its psychological value because of its association to jewelry. Scarcity alone cannot explain its price.
As a practical means of "value", mainly if you also want to move it, it is not very convenient. Here Bitcoin works much better. Bitcoin has many advantages compared to gold when it comes to usability for payments or even for moving parts of your wealth.
For these reasons, I think Bitcoin can surpass gold's "market cap". However, I think gold's market cap has some likelihood to shrink considerably, so a $100,000 Bitcoin could be enough to surpass gold's market cap in the future.