if I have my circle-ci deplying to google with e.g. \'gcloud app deploy app.yaml\' is there a simple way I can pass a dev / prod variable and have it replaced in my app.yml
Sadly there really isn't a good way to do this. I'd actually suggest not using app.yaml for this purpose, and instead using a json file and nconf. That's how we do it in all of our nodejs samples.