config/environments/production.rb

config.action_mailer.delivery_method = :smtp
config.action_mailer.perform_deliveries = true
config.action_mailer.raise_delivery_errors = true

config.action_mailer.smtp_settings = {
  :address              => "smtp.gmail.com",
  :port                 => 587,
  :domain               => 'gmail.com',
  :user_name            => 'account@gmail.com',
  :password             => 'password',
  :authentication       =>  :plain,
  :enable_starttls_auto => true
}
…

config/gitlab.yml

# Gitlab application config file
# Email used for notification
# about new issues, comments
email:
 from: account@gmail.com
  …