ianhanniballake's answer works, but there is a better way to do it. You don't need Google+ sign in to get user info.
Authorize with scope https://www.googleapis.com/auth/userinfo.profile
Make GET request to https://www.googleapis.com/oauth2/v1/userinfo?alt=json
You will get
{
"id": "xx",
"name": "xx",
"given_name": "xx",
"family_name": "xx",
"link": "xx",
"picture": "xx",
"gender": "xx",
"locale": "xx"
}
There are also language specific working codes in the following documentation: Retrieving and Using OAuth 2.0 Credentials.
Have fun!