require 'smtp_tls'

ActionMailer::Base.server_settings = {
  :address => "smtp.gmail.com",
  :port => "587",
  :domain => "localhost.localdomain",
  :authentication => :plain,
  :user_name => "someusername",
  :password => "somepassword"
}

Anatol Pomozov has written a small library using net/smtp and OpenSSL that changes some of net/smtp's methods to allow it to directly talk to secure SMTP servers such as GMail's. He also presents an example of what to do in your Rails application that looks like this:

http://blog.pomozov.info/posts/how-to-send-actionmailer-mails-to-gmailcom.html

Google ActionMailer Fix

Rails