Honestly, I have no idea about Firebase or AppEngine in China. It might entirely be FUD. But, if what you imply is true, then Kubernetes is probably a good solution for your goals.
Run your application on GKE and grow your company. When it comes time to move into China, add a cluster on a cloud provider there.
Run your application on GKE and grow your company. When it comes time to move into China, add a cluster on a cloud provider there.