To forward all of your gmail to another email address (like hanmail or naver), click the gear setting and 'Mail settings.
Then click 'Forwarding
and Pop/IMAP'. Enter the email address you want email forwarded to and
click 'Send email verification instructions.
You will receive an email
at the forwarding address. Click on the link in that email.
Then go back to your gmail settings. Check 'Forward a copy of
incoming mail to' select the forwarding address and 'Add a forwarding