Just about all verified SMTP accounts that are using smtp.gmail.com are targeting port 587 with SSL switched on, but 25 should also work.
Google will also occasionally block any new 3rd party SMTP connections. There’s an area in your Google account you can white-list us, let me find more info on this.
EDIT:
Here it is:
http://community.quickfile.co.uk/t/smtp-settings-5-5-1-authentication-required/1966/15