How would they turn that inside out? What would GCP actually run on if they ran GCP on GCP? Or do you mean run search/gmail/youtube on GCP? Isn't their internal infra basically kubernetes in all but name?
One of the more interesting things I've observed over the past few years is the incremental effort to get AWS teams "lower in the stack" onto native AWS offerings. This makes sense: the less parallel or underlying infrastructure you have to maintain to scale and support the public cloud, the more time and attention you can devote to public services. Many (most?) new AWS services are fully native AWS (including the one I work on :)), and design and operational readiness reviews require justification for not using native AWS rather than the opposite.