how to specify the redirect url for the google sign-in button

Param name is data-redirecturi. Example:

<div class="g-signin2" 
   data-onsuccess="onSignIn"
   data-scope="https://www.googleapis.com/auth/plus.login"
   data-accesstype="offline"
   data-redirecturi="https://www.example.com/redirect_uri"></div>

Note that you don't have to set data-cookiepolicy, it's single_host_origin by default.