With the switch to a new inbox, which is optional, Google has the opportunity to try out new types of ads in Gmail.
The search giant is now experimenting with a new ad unit that mimics the style of a regular inbox entry.
The ads use a different color background, to differentiate them from the regular inbox messages. That's very similar to the way Google has been placing ads in search results for more than a decade now. They're also only visible in the Promotions tab.
The new ads are supposed to be social; they can be favorited or even shared with friends, for whatever reason. Google is probably hoping that the ads will be so relevant that people may think they're useful to their friends as well.
Still, some users may find the new ads annoying. They aren't enabled for everyone, but there's no way to turn them off if you do get them.