How do I delete a GCP organization?

Solution 1:

If you delete your Google Workspace account, it will delete your Organization and all resources associated with it. Therefore, if you want to delete your Organization, you can do so by deleting your Google Workspace account. For Cloud Identity users, cancel all other Google services, then delete your Google account. This is potentially a very damaging action that might be impossible to fully reverse, so it is recommended to only take this action if you are certain there are no resources in active use.

Here is the public documentation:

https://cloud.google.com/resource-manager/docs/creating-managing-organization#deleting_an_organization_resource