I have accidentally deleted my mails in gmail trash folder. How can I retrieve those emails ? I hope I can get a solution for the terrible mistake I made.

You chances of doing this are next to NONE.

As gmail is web based they are gone forever
If you access gmail through a client, like outlook express there maybe a very slim chance of recovery using undelete tools.

What that slim chance I want to know how to recover such mails

I sure g-mail has your email some where since all are scanned, that is why you get advertising on keywords in your email. Also sure g-mail uses some type of RAID backup system it would also be there. Just guessing it would be time consuming getting through channels to get the lost email unless you were government.