To authorize other applications to access domain-wide data of Gmail and Google Drive users, delegate domain-wide authority to the service account.
For instructions on delegating domain-wide authority, go to the Google Developers website, Delegating domain-wide authority to the service account.
Enter the following G Suite, Gmail, and Google drive API scopes that your application will access:
https://mail.google.com/, https://www.googleapis.com/auth/admin.directory.group.member, https://www.googleapis.com/auth/admin.directory.group, https://www.googleapis.com/auth/admin.directory.orgunit, https://www.googleapis.com/auth/admin.directory.user, https://www.googleapis.com/auth/admin.directory.user.alias, https://www.googleapis.com/auth/admin.directory.rolemanagement, https://www.googleapis.com/auth/drive,Note
After you delegate domain-wide authority, you can see the application Client ID. You will need this information when you create an App.