Between iCloud Calendar accounts and Google Calendar accounts this is not quite straightforward to achieve. Because different companies ;)
Just to make it clear, this tutorial is to be able to see iCloud Calendars, not all the calendars from the Mac app Calendar. Because many of those calendars can be from any providers.
1- Go to the macOS Calendar, then select the iCloud Calendar you want to see in GCal for Google Calendar.
2- Check the box Public Calendar.
3- Copy the generated URL.
4- Go back to GCal for Google Calendar.
5- Go to Settings > Add Calendar > from URL.
5- Click "Add Calendar".
Repeat the same steps for all the calendars you want to see.
NB: Please note that this is subscribing, not syncing, which means that you cannot edit the iCloud calendar from within GCal for Google Calendar. Also, changes in iCloud may take up to 48 hours to show up in GCal for Google Calendar.
More on this topic...