Step 1:

The owner of the Google calendar needs to be logged into Google and go to calendar.google.com

Step 2:

Hover over the desired calendar then click the three-dot icon and select “Settings And Sharing”

Step 3: 

Scroll down to “Share with specific people” > Click “Add People” and add the email address of the Google Account that is synced in HighLevel > set the Permissions to “Make changes to events” > click Send to save

The Google calendar should now be in sync with HighLevel