First, go to iCalShare and locate the calendar you want added to your gCal. There are legions of calendars available, so finding one (or more) should be pretty easy. Once you've found one you like, right-click (or control-click) on the "Subscribe" link and copy the link's address to your clipboard (simply left-clicking on the link will launch iCal). Now that you've got it, we're on to step two.
Set yourself up with a gCal calendar. Using a browser other than Safari, go to http://calendar.google.com and get yourself set up. Next, click "Manage Calendars" on the left hand side (see image at right). A new window appears. On the right hand side, click "Add Calendar."
At this point, a new window appears entitled "Add Other Calendar." Click "Public Calendar Address." Now you're presented with a field labeled "Public Calendar Address," and the rest is as simple as pasting the address you grabbed earlier into that field and clicking "Add." Done!