From what I understand TrueNAS uses rclone for Cloud sync. In the case of Google Drive I found following information on their documentation:
When you use rclone with Google drive in its default configuration you are using rclone's client_id. This is shared between all the rclone users. There is a global rate limit on the number of queries per second that each client_id can do set by Google. rclone already has a high quota and I will continue to make sure it is high enough by contacting Google.
It is strongly recommended to use your own client ID as the default rclone ID is heavily used.
I assume that instead of rclone's credentials, iXsystems one is used here. This quote comes from
Making your own client_id and it provides nice guide how to get OAuth Client ID and OAuth Client Secret.
After you follow that, you'll need one more component for TrueNAS - token. For that I have installed rclone on my workstation and ran `rclone config` and went through the wizard, where I entered my newly created client_id and client_secret. In browser I have authrorized access to my account and I ended up having token in the terminal. It is whole json - within curly braces.
Finally I used all these three components to create new Cloud credential in TrueNAS.
First test with this sync works. I'm not sure if that token will be refreshed automatically or at all.