how can i search an gmail id from internet?

This eHow claims to show you how:

http://www.ehow.com/how_5325922_someones-gmail-address.html

It suggest using this site for reverse gmail lookup:

http://www.intelius.com/email-search-name.html

If you know the persons name, you can look them up on Google Profiles:

http://googlesystem.blogspot.com/2008/11/google-profile-search.html