First of all you need to download Pidgin. If it gives error regarding any dependency during the installation check out the project's sourceforge page.
Click on the Add button on the bottom and you will be shown something like this
I have filled almost all the values for my test but you need to change them according to your need.
Protocol: XMPP (earlier used to be Jabber)
Screen Name: Must be filled up with your gmail-id before @gmail.com.
Server: Should be gmail.com.
Resource: Can be anything the default "Home" will also work.
Password: You gmail password goes here.
You can select other options according to your need. It's all on choice. I have selected what I feel are necessary for me.
After you are done with this tab select Advanced Tab. It will look something like shown below:
This is it. Hope you get your g-talk account working with Pidgin 2.0.0 in GNU/Linux.