Gross sales is the total sales done by your business. Suppose you take any products that you sell and obviosuly it can be multiple. For our explanation, I will take one product whose selling price is |$25. Please note that we have to take selling price and not gross price.
Now, in an year, you sold 2000 pieces of the product or you can call as quantity as well. Hence your total gross sales would be your Selling price X Quantity. In this case the total gross price of your product will come out to be $25 X 2000 which is $ 50,000.
This is how you calculate the gross sales.