It's part of a campaign to, "celebrate this connection between jeans and music", the company explained.
Between August 8-31, Gap will give a free iTunes download to store visitors who simply try on a pair of Gap's new autumn range of three jeans for women and one for men.
Print advertisements for the campaign will run in August issues of weekly magazines including US Weekly, People and Entertainment Weekly. TV ads have also been prepared.
Gap Marketing executive vice president Jeff Jones explained: "The relationship between jeans and music is a hallmark of Gap's identity, so we're thrilled to team up with iTunes to bring this connection to life for our customers."