An intensified cyber attacks on google users has persisted since the announcement of state-sponsored cyber attacks.
The attacks affect tens of thousands of users, Google information security manager told the Times. Many have described seeing the Gmail warning in Twitter posts.
If you are among those who have recieved the message inserted by Google displayed below,
(http://asset2.cbsistatic.com/cnwk.1d/i/tim/2012/10/03/Screen_Shot_2012-10-03_at_9.40.19_AM.png)
image by Casey Newton/CNET
then one of the precautions you might want to start with is changing your password and if you haven't enabled two-factor authentication on your Google account, then consider it your second step.
ref:cnet