Question (as someone without domain knowledge): could someone explain what the expected relationship between GDP and total stock market value is? GDP represents

Question (as someone without domain knowledge): could someone explain what the expected relationship between GDP and total stock market value is? GDP represents non-publicly traded, and even non-private activity, while presumably the stock market's valuation is at least somewhat driven by expectations of future growth/profit, rather than current productivity. I don't doubt that there's a relationship of some kind, but what is the simple ratio actually showing?