Google has introduced new update in which Google Plus users who are not added in your contact list of Gmail ID or don't even know your Gmail ID will be able to contact you on your Gmail ID.
This could be termed as merger of Gmail and Google Plus contact but many users may not find it suitable specially spamming may increase a lot.
To block Google Plus users contacting on your Gmail ID:
1) Login into your Gmail account.
2) Go to the settings and locate option "Email via Google+":
4) Don't forget to save the settings.