Ads at the top of the Promotions tab in Gmail inboxes ! It's a new display opportunity to target your audience. You can excpect a better CTR on your ads, because now you can consider making a Gmail-specific campaign.
That way, it's easier to make sure that all of your Gmail ads have the correct targeting
and aren't competing against yours others campaigns in search or display for example.
Gmail ads allow you to target users with sophisticated targeting signals and a high-impact, message-like ad format.
Users are shown a collapsed ad at the top of the Promotions tab in their Gmail account. The collapsed ad opens to
an expanded ad that can be designed to promote a variety of goals (including embedded forms or video).
You can also include links to an external site, app marketplace, or click-to-call.
More about :
https://support.google.com/adwords/answer/6105478?hl=en