Has this changed? When I looked pre-ga the requirements were you need to pay for the CPU 24x7 to attach a GPU so that is not really scaling to zero unless this requirement has changed...
So if you get all your requests in a 2 hours window then that's great. It will scale to zero for rest of the 22 hours.
However, if you get at least one request every 15 mins then you will pay for 24 hours and it is ~3X more expensive then equivalent VM on Google Cloud.