Empathy is another program on Ubuntu which also uses several different chat protocols.
Up in the top bar of 10.04 and 10.10, there is an image of an envelope.
If you click on the envelope, a context menu will pop up and there will be an entry in there that says 'chat' (or 'set up chat accounts' if you haven't used it before)
If you click on this it will start Empathy and you can set up any chat accounts you have. If you've already got some chat accounts set up, you can simply start Empathy, add a new account and enter the details for your google account. Once that's done, you can chat to any of your online contacts on google chat... simple!
Empathy is also integrated into the desktop. Once you've started Empathy you can set your online status (available, away, offline etc) from the top bar of the desktop too!