Before doing sync with GCP you should have done the sync of G Suite, Otherwise, you will miss on the users which get access through G Suite groups. How to sync your organization G Suite checkout link
To do this operation you need to be an admin with Zilla ( Specifically SuperAdmin role ) & admin access with your organization's Google Cloud Platform - GCP ( Specifically, Access Approval Approver equivalent or more privileged role than that ) application.
With the above two things done we can start with GCP sync,