Documentation page 135 

  1. Switch “Salesforce Lightning” view to “Salesforce Classic” view
  2. click on the “Calendar” to view
  3. click on an event
  4. In Google Calendar – click on “settings” icon
  5. in the drop down, click on “settings”
    1. look for “Import & export”
    2. click on “Import”
    3. click on “Select file from your computer”
    4. Choose downloaded .ics file 
    5. Click “Import”
    6. In the Settings view, a message will be prompted after import job is successful.
    7. Refresh the Calendar, imported event will appear on the Calendar