Google Side Configuration.

To configure sync, needed Google Calendar Client Secrets. You can generate it on the page:

To generate "Secrets":

Push “CREATE PROJECT BUTTON"
Enter the name of the application and organization (if your google account is corporate)

If you use a Google account of the organization, your application does not require Google verification.

3. Lets create OAuth Credentials:

Push “Create Credentials" button
Choose Application type, Name and Authorised redirect URIs:
- Web Application
- Calendar for Jira
- YOURJIRAURL/rest/doitbetter-calendar/1.0/googleSync/oauth2callback

And click “Create" button.

4. As a result, you will receive a JSON-file with the Secret necessary for Jira configuration.

Jira Side configuration.

Go to Configuration->Manage apps-> DOITBETTER CALENDAR → Google Calendar settings
Past your Google Calendar Client Secrets from JSON file that you received above.
Click Submit