We published the first GCE Debian 9 Stretch image to the `debian-cloud-testing` project. We will be maintaining this image as best as we can until Stretch becomes Debian Stable and continue to use this project to publish Debian Testing images as time moves on. Let me know if you have any feedback.
To try it out you can list the image with `gcloud`:
`gcloud compute images list --project debian-cloud-testing --no-standard-images`
Or start a basic instance:
`gcloud compute instances create stretch-test --image-project debian-cloud-testing --image-family debian-9`