To add this to your own Google calendar, press the “Google Calendar” button in the lower right corner of the widget.
Alternatively, in Google Calendar, click the “+” button next to the “Add a friend’s calendar” box on the left hand side of the page. Then, click “From URL” and enter the following URL:
https://calendar.google.com/calendar/ical/gf0fm5075ehqquqdsb44uqjlgk%40group.calendar.google.com/public/basic.ics