Retrieving Gross and Net Sales

The square dashboard generates reports that include Gross Sales and Net Sales for a given period.
Is the payments API the only API needed to generate this report?

This doesn’t seem like the best approach given that each Payments API can only retrieve 100 payments at a time.
If this is the only API needed, how do I obtain returns, discounts and comps for a given period?

You will want to use the Orders API for net and gross sales. Within the order are the order objects include fields for line item details, fulfillment details, and order summary data, including the location ID credited with the order. With this information you will be able to calculate the total amount of taxes collected for calculating gross sales and net sales.