Syncing Google Cal and iCal

I have been using Google Cal app and would like to also use my iCal on my iPad, iphone, and MBP. Is there a way to enter an event in iCal and have it show automatically in Google Cal? Do I have to enter each event in separate calendars? When I turn on local calendars, it seems to duplicate all the events.

Use Google via Exchange (google sync) on your iDevices

Setting up Google Sync with your iOS device - Google Mobile Help