You cannot change the runtime configuration of the Kubernetes apiserver in Google Container Engine and by policy alpha APIs are not enabled because they have no official support policy. From https://kubernetes.io/docs/concepts/overview/kubernetes-api/:

Alpha level:

• The version names contain alpha (e.g. v1alpha1).
• May be buggy.
• Enabling the feature may expose bugs.
• Disabled by default.
• Support for feature may be dropped at any time without notice.
• The API may change in incompatible ways in a later software release without notice.
• Recommended for use only in short-lived testing clusters, due to increased risk of bugs and lack of long-term support.

If you want that particular alpha API enabled in Google Container Engine, you can create an alpha cluster.