If you suddenly find that Outlook 2003 is unable to receive Gmails, despite having a valid login (a login which works on the web based Gmail site), then it is likely to be a captcha issue.
Clear captcha, by using link below:
https://accounts.google.com/DisplayUnlockCaptcha
For other Outlook 2003 Gmails issues, use link below:
http://mail.google.com/support/bin/answer.py?answer=75289#
