What version of gsutil are you running?
Also, to be clear: Are you talking about the fact that gsutil checks for available software updates periodically, and if it finds them it then prompts you whether you want to update? Or are you talking about the fact that the gsutil update command asks if you want to perform the update?
If the former, gsutil shouldn't be performing this check/prompting if you are running gsutil from a script not connected to at TTY. If that's not working correctly we'd like to know.
And also, if that's the problem you're having, you can completely disable automated software update checks by setting software_update_check_period=0
in the [GSUtil]
section of your .boto config file.