Depends what stock market you're talking about. The GDP is a flow-variable measure of how much money traded hands in a year in an economy (national). It's suppo

Depends what stock market you're talking about. The GDP is a flow-variable measure of how much money traded hands in a year in an economy (national). It's supposed to function as a measure of yearly production. The market cap of companies within the NYSE, LSE, etc are stock variables. Literally the sums of all the current trading prices times the number of outstanding shares of each company (roughly). There's lots of noise in all those variables. i.e. You probably couldn't fully liquidate every stock of a company at the exact, same price in one transaction. Talking about GDP for a nation can be varying levels of politically manipulated or just noisy from messy data. There's further noise introduced when trying to figure out a GDP for something like the global economy. i.e. taking a ratio of stock market cap to a GDP is more like creating a metric without too much intrinsic theory for why to create the metric.