Have you tried using the test option in Admin panel -> Emails -> Diagnostic?
Have you enabled pop/imap at gmail for your account? I'm not sure that you can use gmails SMTP servers, I've never tried. But I imagine that you would need to check the settings with gmail and use SMTP Auth.
You may want to read this: (the gmail section)
http://www.osticket.com/wiki/Email_Settings(http://www.osticket.com/wiki/Email_Settings)