Bitcoin is a virtual currency that exists only digitally and can be purchased only online whereas gold is a physical assests that you can touch and hold. The difference between is currency part . Bitcoin is more volatile as compared with gold. Bitcoin is referred to as a gold. Both have their own importance on their own.
That is both relevant to each other as well. Bitcoin is virtual but in today's world too many things are virtual, you can hold gold if you want to in your hands but you could also buy gold from your bank account, it would show you owning 10 ounces of gold without ever touching any gold at all, it is just paper, same with fiat as well, you have money digitally, it is in your bank and you can use your credit card instead of actual money and hold the cash in your hands.
Overall, I would say that it is obvious people are using stuff virtually. BUT the fact that crypto is purely digital means it could be used to transact money, buy stuff, sell stuff, work, pay and so forth and that is the biggest difference.
Gold is just good for a pure investment and is definitely centralized. It has been here for decades so preferably, its the most stable investment that even old people have also their own. And its definitely a safe haven investment because its value will keep on increasing but not on a bigger basis.
Bitcoin is not just an investment because its end goal is to become a good currency. Although its not yet totally adopted these days but we can expect that if the world goes more digital, then using bitcoin as a digital currency will eventually be realized as most of the transactions nowadays are becoming virtual. And while gold is centralized, bitcoin is very much decentralized so it cannot be controlled by any entity. Bitcoin has becoming a gold investment in the years to come as its adoption is increasing, then its value will also become bigger too.