This is well documented already. Asterisk 1.8 Documentation Calling using Google
"In your Google account, you'll want to change the Chat setting from the default of "Automatically allow people that I communicate with often to chat with me and see when I'm online" to the second option of "Only allow people that I've explicitly approved to chat with me and see when I'm online."
Also one would be well advised not to use their main Google account for such purposes. I recently configured Skype for Asterisk on my own box. Skype forces this policy for SFA and Skype Connect. I can see the wisdom in not using a personal account for integration into a PBX.