Before you can add a Calendar to your guild you will need to get the ID from the Google website. Here are a few steps on how to get this ID manually from Google Calendar.
Step 1: On the Google Calendar website click the gear in the top right and press settings.
Step 2: "Calendars" in the top left. Then click the name of the Calendar you want to use.
Step 3: Find "Calendar Address" and then copy your "Calendar ID"