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.

Also, honestly, I even think the day bitcoin becomes a global reserve is still a long way off, I don't see it happening in this decade or the next. Look at what's happening now, how central banks around the world are treating bitcoin. They don't need it like we think, the FED has publicly stated that they have no intention of adding bitcoin to the federal reserve, or the ECB has even outright criticized bitcoin and considers it only a speculative asset, and not suitable to be a currency or reserve asset.
So let's forget about bitcoin replace gold as the global reserve anytime soon.