1. Under Outgoing mail, set the SMTP server to smtp.google.com.
2. Set the your username is [email protected]
and make sure "Use username and password" is checked.
3. Also check off "TLS" under "Use secure connection."
Gmail automatically rewrites the "from" line of any e-mail you send via their SMTP gateway to your Gmail address, and it overrides any Reply-To settings you may have in your e-mail software in favor of the one in Gmail's web interface.
This is not a great solution for marketers trying to improve deliverability, and using it for marketing probably violates Gmail's terms of service.
Furthermore, if you are mailing to an established list, your recipients are probably looking for your e-mail to originate from your company's domain, and not from Gmail. That's a good way to wind up in the junk folder.