If you decide that you don’t need your Gmail address anymore, and you’re sure you won’t want your username in the future, you can simply remove Gmail from your Google Account.
- Click the gear icon in the upper-right corner of Gmail, then Mail settings, and open the Accounts and Import tab.
- Click Other Google Account settings in the ‘Change account settings’ section.
- Click Edit in the ‘My products’ section.(If you use Google+, click Visit the previous version of the Google accounts screen link at the bottom of the page to see the ‘My products’ section)
- Click Remove Gmail permanently.
Please be aware, residual copies of deleted messages and accounts may take up to 60 days to be deleted from our active servers and may remain in our backup systems.
If you delete your Gmail address but wish to have it back, we work to help you recover your deleted accounts whenever possible. However, within a few weeks’ time, accounts are usually no longer retrievable. Note that a successful recovery will only recover the username associated with the account. [source]