By default the Geddy.js app will listen on port 4000 in production mode, so it must be updated to listen on the port specified by the PORT environment variable available in the Google App Engine environment.
Open config/production.js and change port: 4000 to port: process.env.PORT || 4000.
Now run the following command to deploy your app:
gcloud app deploy
Visit http://YOUR_PROJECT_ID.appspot.com to see the deployed app.
Note: When the the Geddy.js app is deployed it will automatically switch to production mode and listen on the correct port.