I am using phpmailer to send email. I need to know how to hide or mask sender email address
I assume you already know how to use the class you are talking about.
You probably have some variable for sending email, like
var $From = "someguy@whatever.com";
you can type whatever you want into that email address. Gmail dont care what email things is sent from.
And no, this dosent sound very legit.
One more thing: Gmail requires a gmail account to relay mails. Its no problem, it wont be visible.