If you'd like to see a report of the total gift cards redeemed over a period of time, follow these steps:
On the Filters tab, select the date range that you would like to report on.
On the Metrics tab, add this metric to the report: "Payments » Net Payments."
On the Groups tab of reporting, add the "Payments » Payment Type" group to the report.
This report will show you a breakdown of payments by payment type, including the total of gift cards redeemed during the period.