The Market Capitalization of Gold is currently arround $17.919 T.
This value was obtained by multiplying the current gold price ($2,668 per ounce) with the world's above ground gold reserves.
The amount of above ground reserves for Gold are estimated to be around 208,874 metric tonnes according to the World Gold Council (End-2021). Note that the estimated above the ground Gold reserves can vary by up to 20% from one source to another.
As a consequence it is safe to say that the the current Market Cap of Gold is between $14.335 T and $21.502 T.
See also: The Market Capitalization of Silver