I am not familiar with GCE so I can be absolutely wrong, but my way of thinking is that any time anyone says something can run the open source version anywhere, true, but when it comes to offering a paid service, making some technology native to the platform, means there can be difference such as customized APIs or customized features which may never get backport into the community/open source version.
That is very clearly not how they are working with Kubernetes. They are doing it entirely in the open and even recently hired one of their huge community contributors (shoutout to Kelsey Hightower) to the team. If they were doing an "open core" version, my money would be on Kelsey speaking out against it. Besides, given their goal of making the cloud a commodity, they can't do what you say.
