They measure different things:

• Gross Volume (Order Metrics) sums the full order total – products, tax, fees and tips – but only for orders that are Paid or Partially Refunded.
• Gross Sales (Gross Sales report) sums product subtotals before discounts for all orders regardless of status.

Order Metrics is also limited to the kitchen location you are working in. Pick the report that matches the question: Gross Volume for "how much did customers pay", Gross Sales for "what was the list value of what they ordered".