That's 50% off retail at Gap stores, not valid online, outlet stores, factory stores, or any other stores not relating to Gap. Certificate expires on November 19th. You can also use this certificate on sale items. Yay!
Just go HERE to make the purchase if you are interested. If you cannot get into the site, just type http://www.groupon.com/. If you see an error screen come up due to a high demand on purchasing these certificates. Just keep on refreshing the page or retype the website address until you get in. I had to do this three times before I was able to get in.
If you want learn more about Groupon and understand how it works, go HERE.