IP ranges for Google APIs not including those available to GCP customers
Google provides two lists of IP ranges; an exhaustive list of all Google IP ranges and a list of IP ranges available to their customers on Google Cloud Platform. They do not provide a list of Google-only ranges but instead suggest you compute it subtractively. This repo contains a script to do so and a job that runs daily to maintain a publicly available list of these ranges.
It probably makes more sense to do this yourself, as it would take almost as much code to determine if this list is trustworthy. This is mostly for my personal use.