You're getting the 500 error because Google is blocking your heroku ip from accessing your account. They aren't sure it's you.
You need to change your activity settings to authorize that ip/domain.
Read this: https://support.google.com/accounts/answer/1144110?hl=en&ref_topic=2401957
Also, its a good idea to read your logs when debugging these kinds of errors. Rails.logger.debug results
could shed some light.