You are prompted for authentication, and your credentials are not accepted. You receive the following error message in Outlook:
Your IMAP server wants to alert you to the following: Please log in via your web browser:
http://support.google.com/mail/accounts/bing/answer.py?answer=78754 (Failure)
This issue occurs when you try to sync to a Gmail account after July 15, 2014 and Basic Authentication has not been enabled for your Gmail account.
Google has increased its security measures to block access to Google accounts after July 15, 2014 if those accounts are being set up or synced in apps and on devices that use Basic Authentication.
To resolve this problem, enable Basic Authentication for your Gmail account. Go to the following Google website, and then select Enable when you are prompted about whether to let less secure apps access your Gmail account:
https://www.google.com/settings/security/lesssecureapps
For additional security, consider enabling Google’s two-step verification and app passwords as documented at the following web pages: