I have seen the statement that "outbound traffic from GCP to other Google products (such as Google Drive) is free". I would like to confirm if this statement is true？
You will find a list of all the use case for GCP to non-GCP product traffic costs at this page. Section "VM-to-Google service".
But your initial statement is correct, traffic from a VM in GCP to Drive is free regardless of if the VM has a public or private IP. That's because that traffic never leaves Google backbone