Step 1: Install the Google data python API client by following the instructions here:

  http://code.google.com/p/google-api-python-client/wiki/Installation

  Hints:
    In Ubuntu, you may need to install python-setuptools: sudo apt-get install python-setuptools

    Also, in my Ubuntu install, the permissions in Google's gdata-2.0.14.zip were all messed up. 
    Just run chmod a+x -R *.py from within the gdata-2.0.14 directory if this happens to you.

Step 2a: Go to your google calendar page, and click the little gear in the top-right, and choose 'Calendar Settings'
      b: Click on "Calendars" (between General and Mobile Setup at the top)
      c: Choose the calendar you want to display, and click the "Shared:Edit Settings" link
      d: Click the "Calendar Details" link (To the left of "Share this Calendar" at the top)
      e: In the "Private Address" section, click on the orange XML button
         You should be presented with some link like:
         https://www.google.com/calendar/feeds/your.email%40gmail.com/private-somerandomstringgeneratedbygoogle/basic
      f: Copy down the private-somerandomstringgeneratedbygoogle part for use in the next step

Step 3: Create a ~/.calgrabrc file that looks like this (without the ------es)
------------------------------------------------------
username: your.email@gmail.com
visibility: private-somerandomstringgeneratedbygoogle
------------------------------------------------------

Step 4: Run ./calgrab!