In Google Container Engine the master is managed for you and kept running by Google. According to the SLA for Google Container Engine the master should be available at least 99.5% of the time.
In addition to what Robert Bailey said about GKE keeping the master available for you, it's worth noting that Kubernetes / GKE clusters are designed (and tested) to continue operating properly in the presence of failures. If the master is unavailable, you temporarily lose the ability change what's running in the cluster (i.e. schedule new work or modify existing resources), but everything that's already running will continue working properly.
