Last month Google updated a similar thread on blog where MITM (Man in the middle attack) was attempted.
Some precautions recommended by Google is as follows
- Change your Gmail password.
- Verify your account recovery options. e.g second mail address, phone numbers, and other info that is filled to use in account recovery time.
- Review websites that is allowed to access your account.
- Check for suspicious forwarders/remote smtp.
- Be smart to warnings/popups appears.