Hi,

After creating the API for Google calendar (please refer to this documentation:

 https://www.odoo.com/documentation/saas-16.3/applications/productivity/calendar/google.html)

Add the user's email under "Developer contact information". You can add multiple emails under this.Don't forget to give proper Authorized redirect URIs as well. It will be your_domain/google_account/authentication

After that go to login as the user you want and go to the calendar module and click sync google calendar option. 

Give the permissions for google calendar to connect with odoo and your calendar will be synced. 
To remove the synchronisation, you can deactivate it from User -> Calendar tab and choose RESET ACCOUNT option.

Hope it helps