Check your settings in gmail (see picture).

Already Checked the issue exactly is that thunderbird downloading only new emails not all old emails from Gmail. So please tell how to download all email in thunderbird.

If you have enabled POP for all mail in gmail settings, try renaming the popstate.dat file in the Mail/pop.gmail.com subfolder of the profile folder, with TB closed. When TB is restarted, it should download all mail that is in the Inbox on the server.