Amazon is, of course, the biggest e-tailer in the world—and, with Prime Video, it’s also one of the biggest streaming services. And since it started running ads on Prime Video content for all viewers last year, it also has an ever-expanding amount of digital ad inventory for marketers who want to run commercials alongside the originals it pays hundreds of millions of dollars to produce.

Amazon can tell those marketers if their ads drive sales on its platform. What it doesn’t do, however, is tell them whether their ads drive sales from retail giants like Walmart and Target. That info is traditionally available only to the retail giants themselves, kept in their “walled gardens” of data.

But retail media tech companies Stackline and Gigi have now signed a partnership to access it—and share it with marketers.

Per Adweek, Gigi is a startup that helps brands place TV ads on Amazon-,...