If bitcoin becomes the global reserve, nothing will happen to gold.
I mean in a sense that gold will still be and always be gold but we can just accept the fact that by the time BTC starting to get known, that have affected the the demand that supposed to be going for gold.
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.
And BTC is also like that. It is not limited as an asset as well but it also functions as a currency. That makes it even more valuable. AFAIK there are countries or governments already that started using BTC as their reserves. Time will come that all of them are now doing that but I think that is only the formal time for us to call BTC as a global reserve. We can't tell the exact time for this. It can be long or it can be short depending on how things go with BTC or on the factors that surrounds it.