Google continues to try and cram its users into Google Plus, its also-ran social network. The latest move allows people who don't have your Gmail address to send email to your Gmail account by using your Google Plus ID. I have a Gmail account that's associated with my Android devices and the last thing I want is for people to start sending email there. Thankfully, there's a way to opt out (though it would have been much better if it was opt-in). Tl;dr: Gmail -> Settings -> Email via Google+ -> Off.