Now a days most of the hackers are targeting gmail accounts dude to the no of services offered by the google
if hacker change the secret question and secondary email to new ones. Once these two things are changed, you cannot get back your account through the forgot password tool.
then follow these steps
Gmail has a special page to report such incidents when someone hacks into your account.
check it here :https://services.google.com/inquiry/gmail_security1.
The Gmail Team considers numerous data points when determining original account ownership in a hijacking case. In addition to the information requested on the form, you can also include the following information to help them process your claim.
The email addresses of up to 3 Frequently Mailed Contacts
The names of up to 3 Labels
Any other Google services you used with this account and the date you started using each one
Your ip-address
You can also resubmit the form later if you find any additional information that would help them investigate. This process is said to take about 4-10 days.