I think it has to be done through google+
click google+ on home screen, then gives you option apps to click......talk.....then your in gmail chat.
Talk? Or Messenger. Messenger in Google+ is not Google Talk, at least not yet.
They need to put those two together with GVoice to make one app.
This thread is a perfect example of why Google needs to unify all of these apps into on, and make all the messages cross populate accordingly.
I think it has to be done through google+
click google+ on home screen, then gives you option apps to click......talk.....then your in gmail chat.