With this howto, you can add a Google Calendar by generating an URL you can use in the “web” widget for displaying it:
Now, from the generated URL, like this example:
<iframe src="https://calendar.google.com/calendar/embed?height=600&wkst=2&mode=AGENDA&title=Test&src=xxxxxxxxxxxxxxxxxxxxxx&color=%23F4511E" style="border:solid 1px #777" width="800" height="600" frameborder="0" scrolling="no"></iframe>
remove the iframe parts, so you end up with just this part:
https://calendar.google.com/calendar/embed?height=600&wkst=2&mode=AGENDA&title=Test&src=xxxxxxxxxxxxxxxxxxxxxx&color=%23F4511E
When you paste that URL into the “web” widget,
it can look like this:
