As an NGO, we created a Google brand account some years ago but unfortunately the owner of the account left the organization and we can't contact him. So we are still able to manage our youtube channel but we are not able to get full control over the account. How could we reclaim the full ownership of our account ?
Google-Cloud-Platform specific use cases should be reported on Google Issue Tracker.
I have already created this issue for you so the right team would handle it for you.
As my colleague indicated on this issue, this should be handled by YouTube Support.
For this, you would need to submit an new question.