It happens when many time with almost most us when we are busy with some office work and a gtalk friend pings you but you cant respond as you are so much messed up with the work you are doing. So, it could be great if Gtalk itself has some auto reply which it does not provide.
But in order to solve this a developer named shobankr made a small software which can integrate itself with the gtalk installed on your computer lets you auto reply a custom text to your gtalk buddies when you are away.
So when ever some of your friend pings you on gtalk and you are away or cant respond this small app will auto respond to your gtalk friend with the custom text you had set, like the one shown below.
Note: Although the link you can see in the above image with the reply message is being removed by the developer from the application which make this application more cleaner and useful.