This chart shows the seasonality factors that each quarter of GDP can be multiplied by to remove repeating seasonal variation. These seasonality factors are calculated by dividing U.S. GDP - Quarterly by St. Louis Federal Reserve Series GDP (same as U.S. GDP - Quarterly SAAR).
mccormick-charts.com: U.S. GDP - Quarterly, U.S. GDP - Quarterly SAAR
Wikipedia: Gross Domestic Product
Other links: St. Louis Federal Reserve Series GDP