The new feature also takes advantage of the new Gmail inbox buckets that Google announced back in May. When someone in your circles emails you, the conversation will appear in the “Primary” tab. If you don’t have them in your circles, their messages will be filtered into “Social.” 

This is the latest push to deeply integrate Google’s not-so-social network across all platforms. Last year, the company implemented Google+ on YouTube, forcing all commenters to first sign up for a Google+ account.