Is there a Google Calendar Mac Desktop App?
Yes, the built-in Calendar app lets you add them. 

For your Mac:


On your computer, open Calendar 
In the top left corner of your screen, click Calendar > Preferences.
Click the Accounts tab. On the left side of the Accounts tab, click +.
Select Google > Continue.
Enter your Gmail address, password, and verification code (if you have one).
Click Accept.
On the Accounts tab, choose how often you want Apple Calendar and Google Calendar to sync.


For more information, see https://support.google.com/calendar/answer/99358