Here are the settings you'll need to use when utilizing Gmail in ActionMailer. Simply open up the config file for your production environment and add the code below. Then mail should send out via Gmail. If you need to send out email in the development or test environments, edit those files as well.
config/environments/production.rb:

config.action_mailer.delivery_method = :smtp
config.action_mailer.smtp_settings = {
  address:              'smtp.gmail.com',
  port:                 587,
  domain:               'example.com',
  user_name:            '<username>',
  password:             '<password>',
  authentication:       'plain',
  enable_starttls_auto: true
}