The price of gold if most often measured in Troy Ounces (Oz) and the most current price (latest traded) is called the "spot" price. Gold can also be measured in gram and kilogrames but that is not as common. Financial instruments related to gold could have other prices as they could be priced in other ways.
There are tons of different ways that gold trades: ETFs, options, futures, tokenized gold on a blockchain etc. To make all this possible, gold is often stored in vaults and financial products are created from that backed gold. Gold could also be traded in physical form such as coins, raw gold or jewlery and have additional value because of a special design or rarity of a coin.
Today gold is trades on most of the worlds exchanges. Several banks and institutions provide their own gold products.
Investing and speculating in the price of gold is very common. Gold is considered quite stable asset and has historically worked quite good as a hedge to the stock market during recessions. You can own gold in paper format (financial instruments) or in physical form (coins, jewlery etc.).
There are severall estimates for how much gold there is in existance, but the truth is, no one knows. Gold is also mined, which increases supply, gold is also lost, which decreases the supply.