mail_service.rb

require 'google/apis/gmail_v1'
require 'uri'
require 'mail'

Gmail = Google::Apis::GmailV1

class MailService

  def initialize(params)
    @params = params
  end

  def call
    message = Gmail::Message.new
    service = Gmail::GmailService.new
    message.raw = "VG86IGJkam9obnNvOEBnbWFpbC5jb20KRnJvbTogYnJhZGxleS5kYXZpZC5qb2huc29uQGdtYWlsLmNvbQpTdWJqZWN0OiBUZXN0IHN1YmplY3QKClRlc3QgYm9keQ"
    service.request_options.authorization = current_user.token.fresh_token
    result = service.send_user_message(current_user.email, message)
  end

end

omniauth.rb

Rails.application.config.middleware.use OmniAuth::Builder do
  provider :google_oauth2, ENV["GOOGLE_CLIENT_ID"], ENV["GOOGLE_CLIENT_SECRET"],
  {
    scope: ['email', 'https://www.googleapis.com/auth/gmail.modify'],
    hd: ENV["GOOGLE_DOMAIN"],
    access_type: 'offline'
  }
end