Gross volume is a backwards-looking metric that shows the total amount of revenue a newsletter has made over a given period. It's a good way to see how much revenue a newsletter has made over time, and is a good metric to use when comparing newsletters.
It's not a perfect proxy for a newsletter's profitability: for instance, gross volume overvalues one-time payments and annual subscriptions. But, especially if you're curious about cash flow and how much you are going to see in your bank account, it's a good metric to use.