The Gravity Forms pricing fields are available on the form builder and can be used to add product and pricing to a form. Gravity Forms is not a shopping cart but it can be used to pass data to a shopping cart or payment system like Paypal or ejunkie.
To use the form to pass the data to a shopping cart you can set up a redirect and build a query string into the url. Check the pass data via query string box and put a ? at the end of the base url then set up the data by giving it labels like "price=" then use the drop down selector choose what form field data goes after each label. Between data sets enter a &.
This is what the form will look like for the user.
This is what your redirect url will look like when the user clicks submit. And your shopping cart will have to be set up to use the data to populate whatever fields it needs.
You can also find more information in the Gravity Forms Documentation and in the Gravity Forms Support Forums.