I have never used the Google calendar. However, I believe you need to make the Pi a client to use this. I would suggest reading :
https://developers.google.com/gdata/articles/python_client_lib
The first part of this page is just information to you. The Pi runs Linux and it’s distributions come with Python already. Therefore, you just need to check that your Python version is 2.5 or higher. (I am at work now, so no Pi access for me to check the version.) If you at are Python 2.5 or higher, then you can assume that the dependencies are already installed. Dependencies is just a fancy term for “other stuff you also need” . The go to the Installing Google Data Library and start those steps on the Pi.
For all the above comments, I am assuming that you want to place sprinkler schedules on your Google calendar and have OpenSprinkler follow that schedule. To do that, the Pi has to be running something that communicates with said Google Calendar. The foundation of this appears to be Python scripts running on the Pi and the building block is the Google Data Library.