auth_step4.rb

# config/initializers/devise.rb
# for web
config.omniauth :google_oauth2, ENV['GOOGLE_CLIENT_ID'], ENV['GOOGLE_CLIENT_SECRET'],
  { access_type: "offline",
    prompt: "consent",
    select_account: true,
    provider_ignores_state: true,
    scope: "plus.profile.emails.read,calendar,contacts" }

# for api
config.omniauth :google_oauth2, ENV['GOOGLE_CLIENT_ID'], ENV['GOOGLE_CLIENT_SECRET'],
{
  name: "google_oauth2_for_api",
  access_type: "offline",
  prompt: "consent",
  select_account: true,
  provider_ignores_state: true,
  scope: "plus.profile.emails.read,calendar,contacts"
}

# callback controller
class CallbacksController < Devise::OmniauthCallbacksController
  include Devise::Controllers::Rememberable

  def google_oauth2
    outcome = ::Users::FromOmniauth.run(auth: request.env["omniauth.auth"])

    if outcome.valid?
      user = outcome.result
      remember_me(user)
      sign_in(user)
      redirect_to root_path
    else
      redirect_to new_user_registration_url
    end
  end

  def google_oauth2_for_api
    outcome = ::Users::FromOmniauth.run(auth: request.env["omniauth.auth"])

    if outcome.valid?
      user = outcome.result

      auth_token = user.generate_auth_token # because I use https://github.com/gonzalo-bulnes/simple_token_authentication
      render json: {
        success: true,
        auth_token: auth_token,
        auth_email: user.email
      }
    else
      render json: { success: false, message: "Invalid credentials"}, status: :unprocessable_entity
    end
  end
end