google API for authentication

First you need to check E-mail in Database. Please find below links:

http://www.c-sharpcorner.com/UploadFile/20abe2/how-to-check-user-name-or-email-availability-using-Asp-Net/

http://www.dotnetfunda.com/articles/show/2297/how-to-check-whether-username-exists-in-database-without-refreshing-pa

Secondly, once user is verified in your Database, you need to validate through Google API. Here are the links:

http://www.c-sharpcorner.com/UploadFile/vdtrip/google-plus-authentication-in-Asp-Net-and-C-Sharp/

http://www.aspsnippets.com/Articles/Login-with-Google-Account-API-in-ASPNet-and-get-Google-Plus-Profile-details.aspx