Earlier this summer, Google introduced ads in Gmail that look similar to regular emails and appear at the top of the Promotions tab when the tabbed in-box is activated. When you click on one of these ads, it opens much like a typical email message. Now when you star one of these ads, it will be “Saved to your Promotions inbox”, but more significantly, and unstated in Google’s pop-up message shown below, it will also be added to your Primary tab and display as a standard email.
In the Primary tab, you’ll notice the ad also now appears as a standard email message without any “Ad” label.
As others have noted, when you open one of these in standard message format — from either the Primary or Promotions tab — the Dismiss and Forward options are gone and the standard action icons are available.
In message format, the alert that you’ll be submitting information to the advertiser is removed, and the sender’s email address is displayed as email@example.com. In testing, deleting the message (or marking it as spam) doesn’t keep it from continuing to show as an ad in the Promotions tab.
It’s likely advertisers are charged per on a CPC basis when users star an ad, but advertisers pay only for the initial action a user takes. These email-like ads are part of Google’s beta called Gmail Sponsored Promotions.