If bitcoin becomes the global reserve, nothing will happen to gold. Because gold is not only an investment asset but also an important material in most industries and the demand for gold will not be affected just because of a digital asset like bitcoin.
Demand for gold as a material in industry is too overblown, probably only take 7% of the total demand. The demand for investment and jewelry surpass it by significant margin and even jewelry is just another investment.
Though i will agree that nothing will change if bitcoin become global reserve, we just gonna have 2 asset that can be used for global reserve and thats it.
Most likely gonna take some of gold demand but thats about it. Most people will definitely diversify between the two though.