This article will lead you through the steps of adding a Google calendar account to Calendars by Readdle for Mac. To learn how to do this on iOS, please follow the link.
To sync Calendars for Mac with a Google account, tap Google when launching Calendars for the first time, then follow the onscreen instructions. And here's how you can add a Google account later on:
Tip: You can use the ⌘, shortcut to access the Calendars settings.