Getting “Net::HTTPError: Forbidden (403)”

Question

While syncing contacts (fetching contacts from Google or updating a contact from Google), I am getting Net::HTTPError: Forbidden (403).

Would like to know in which cases this exception can arise. Any url that explains this error would help us to debug this issue and analyze different scenarios in which this error can be raised.

Overview of our application's google integration:

1. Our application is built on Ruby on Rails (3.2.18).
2. We are using google-contacts gem (https://github.com/varunlalan/google-contacts) for syncing contacts.
3. We authenticate user using omniauth-google-oauth2 gem (https://github.com/zquestz/omniauth-google-oauth2).
4. OAuth 2 scopes include - userinfo.email, userinfo.profile, https://www.google.com/m8/feeds/

Any help or inputs would be highly appreciated.

Thanks.