I assumed Google Cloud and Drive had the same availability garanties. Searching “Google Drive availability” on Google return the cloud availability page as 1s result. But you’re probably right, there’s never a SLA for free services.
If looking at the page for Google Drive’s paid version one can see some features and claims about the storage’s quality, emphasis mine:
Store, access, and share your files in one secure place
With centralized administration, data loss prevention, and Vault for Drive, you can easily manage users and file sharing to help meet data compliance needs.