According to this blog post Kubernetes 1.6 should be available on Google Container Engine:

Today we started to make Kubernetes 1.6 available to Google Container Engine customers.

But how do I enable it? The gcloud seems to think only 1.5.6 is available:

$ gcloud container get-server-config --zone us-central1-a
Fetching server config for us-central1-a
defaultClusterVersion: 1.5.6
defaultImageType: COS
validImageTypes:
- CONTAINER_VM
- COS
validMasterVersions:
- 1.5.6
validNodeVersions:
- 1.5.6
- 1.5.4
- 1.4.9
- 1.4.8
- 1.3.10
- 1.2.7


$ gcloud container get-server-config --zone europe-west1-d
Fetching server config for europe-west1-d
defaultClusterVersion: 1.5.6
defaultImageType: COS
validImageTypes:
- COS
- CONTAINER_VM
validMasterVersions:
- 1.5.6
validNodeVersions:
- 1.5.6
- 1.5.4
- 1.4.9
- 1.4.8
- 1.3.10
- 1.2.7