GCP has different SLAs for each product, but the ones I've seen are per-customer. The details vary by service, but they generally define downtime, like x% errors for y% amount of time, and then have financial penalties for z% downtime[1]. There are sometimes clauses requiring retries with exponential backoff[2].
Their SLAs are short and readable. It is worth reading a few of them, especially if you have a SaaS and are thinking about your own SLAs. Just search Google for [google $product sla] to find them.