Google doesn't actually offer Zanzibar as a product/service (in GCP or otherwise) to customers. However, they do use it internally to manage permissions across their various products (Google Docs, Drive, YouTube, etc.) and have had a lot of success doing so. Because of that, there are many open source implementations of Zanzibar out there (as others have commented). Warrant also maintains our own open source implementation of Zanzibar[1] which powers our managed cloud offering, Warrant Cloud[2].
[1] https://github.com/warrant-dev/warrant
[2] https://warrant.dev