application.rb

...

# Require our custom warden strategy for Google oauth
require 'warden/o_auth/strategy/google'

authentication_failure.rb

class Warden::AuthenticationFailureApp < Devise::FailureApp

  # We have a failed login attempt, perhaps it's a Google login with a new user that must be created
  def respond
    # HACK: We set @scope to user here to make devise happy
    @scope = :user
    
    # Do we have a Google Access token?
    if access_token = (((env['warden.options'] || {})[:oauth] || {})[:google] || {})[:access_token]

      # Get email of authenticated user
      authenticated_email = ActiveSupport::JSON.decode(access_token.get('https://www.googleapis.com/userinfo/email?alt=json').body)['data']['email']
      
        # Find or create user
        user = User.find_or_create_by_email(authenticated_email)
      
        # Sign in the user and redirect
        env['warden'].set_user(user)
        redirect_to root_path
      else
        super
      end
    else
      super
    end
  end

end

devise.rb

Devise.setup do |config|
  config.warden do |manager|
    manager.oauth(:google) do |google|
      google.consumer_key  = ApplicationSettings::GoogleApps::DOMAIN
      google.consumer_secret = ApplicationSettings::GoogleApps::CONSUMER_SECRET
      google.options(
        :site => "https://www.google.com",
        :request_token_path => "/accounts/OAuthGetRequestToken?hd=#{ApplicationSettings::GoogleApps::DOMAIN}",
        :access_token_path => "/accounts/OAuthGetAccessToken?hd=#{ApplicationSettings::GoogleApps::DOMAIN}",
        :authorize_path => "/accounts/OAuthAuthorizeToken?hd=#{ApplicationSettings::GoogleApps::DOMAIN}"
      )
    end
    manager.default_strategies(:scope => :user).unshift :google_oauth
    manager.failure_app = Warden::AuthenticationFailureApp
  end
end

Gemfile

...

gem 'devise'
gem 'warden_oauth'

google.rb

class Warden::OAuth::Strategy::Google < Warden::OAuth::Strategy

  # Override request token, passing in the scopes we need, in this case the email of the authenticated user
  # http://code.google.com/intl/da/apis/accounts/docs/OAuth_ref.html#RequestToken
  # http://code.google.com/intl/da/apis/apps/profiles/developers_guide_protocol.html
  # http://code.google.com/intl/da/apis/gdata/faq.html#AuthScopes
  def request_token
    host_with_port = Warden::OAuth::Utils.host_with_port(request)
    @request_token ||= consumer.get_request_token({:oauth_callback => host_with_port}, {:scope => "https://www.googleapis.com/auth/userinfo#email"})
  end
  
end