Export all calendars from Gmail
1. On your computer, open Google Calendar. You can't export your calendar from the Google Calendar app.
2. At the top right, click Settings 
Settings.
Import Calendar to Office 365
1. Login to Office 365
2. Browse https://outlook.office.com/calendar/view/month
3. Click Add Calendar as shown in the below image
4. Click the 'Upload from file' and browse the extracted file as shown below
5. Select Calendar and click the Import button.
