All the documentation on Gmail account security (eg compromised account)

From the http://gmailaccountrecovery.blogspot.com/

If you're suspicious about the safety of your account, have a look at these two resources:

How to re-secure an account that was compromised:

http://gmailaccountrecovery.blogspot.com/#security ("When you reclaim Your Account")

Account Security Checklist

https://support.google.com/mail/checklist/2986618?rd=1