I am using ActionMailer and in my mailer model, I have a from set like this
default :from => \"from@example.org\"
In my environment.rb,
Gmail doesn't let you send as a different user -- it will always make the "From" address the account you're using to sign on (this is for security/spam prevention I believe, that way you can't spoof someone else's address).
Assuming you own the e-mail address(es) you want to send from, there's a way through your Google account to allow sending from those e-mail address(es). Go into the Gmail options, click the "Accounts" section, and click "Add another e-mail address you own."
It appears that now it's possible to send emails with a different sender through gmail's SMTP.
I've listed my way to do it here:
Send an email through gmail but configure a different email address