Google Cloud's managed Kubernetes service. Google created Kubernetes, and GKE offers it as a managed service with automatic upgrades, Autopilot mode, and low-cost Spot nodes.
In CDL it's the container orchestration service. Equivalent to AKS (Azure) and EKS (AWS). Since Google created Kubernetes, GKE has deeper integration with the Kubernetes ecosystem.
Google Kubernetes Engine (GKE) is a concept that can show up on the exam. Check the full guide to see it in context.