1. Enable POP in Gmail.
Goto --> Your Gmail account --> Settings --> Forwarding and POP/IMAP and set the settings as below. (Set to enable POP for emails from now on and after save you should see POP is enabled)
2.Setup Gmail account settings as Email Alert settings in TFS 2012.
Open Team Foundation Administration Console --> Application Tier --> Alert Setting
and set the settings as below example. It should be your Gmail account and password :)
And now all is setup and your TFS users can subscribe for alerts in TFS 2012 and will get an email from your TFS Gmail account that you set in above.