I have learned that there are many ways to sign in app with Google Account.
Here are two ways:
AccountManager.java
Google+ Api
AccountManager
You can get the Email address via the Google+ API. You need to add the appropriate scopes and permission.
The scopes are
Scopes.PROFILE
Scopes.EMAIL
And you need the following permission
android.permission.GET_ACCOUNTS
Then you can retrieve the Email address with the AccountApi
Plus.AccountApi.getAccountName(googleApiClient)