I managed to deploy correctly a react app to google app engine.
The build folder is correctly served from app engine, but the deployment contains also all the other files.
How can I deploy only the build folder?
You can add all the files and folder you do not want to deploy to the .gcloudignore file. It's very similar to .gitignore for git. More on .gcloudignore here: https://cloud.google.com/sdk/gcloud/reference/topic/gcloudignore