By default there is no option of making the status in GTalk as idle, You can appear Online , Busy and Invisible but there is no option to appear Idle .You can appear idle by using GalwaysIdle program.
GalwaysIdle is a freeware application which enables you to set your status message as ‘idle’ with just single click.Galwayidle helps you to Control you Gtalk Status by appearing permanent always idle even if you are available in yours system and also helps you to appear Never idle even if you are not available on yours system.
Download GalwaysIdle , Once you have installed the application ,Right click the GTalk on the taskbar and you will be able to see 3 more options- “Normal Idle”, “Always Idle” and “Never Idle”.
Selecting ‘always idle’ lets you appear away even when you are logged in and doing some other work.
‘Never idle’ lets you appear to be at your computer even when you’re far away.