What do you mean? App Engine supports many versions of your application running simultaneously. You just deploy to a new version and migrate traffic (slowly or quickly, your choice).

For those looking for a more automated approach for doing things like blue/green, take a look at Spinnaker (http://spinnaker.io). Here's a quick video for App Engine - https://youtu.be/pOPgTHz_nYw

Disclaimer, I work on the project.

We don't have a deployment strategy named "blue/green" for the App Engine provider for various reasons, but do provide stages such as "deploy", "disable", "enable", "destroy", as well as setting traffic split percentages that you can string together in a pipeline.

Edit: okay, no markdown on HN