I just wanted to add 3 things that you can add to this tutorial:

• Don’t forget to navigate to the firebase web console and enable Google Sign-in under “Authentication” > “Sign-in Method” -tab.
• Add the following under your dependancies in the package.json file in your function folder if you are getting an error with lint: “lint”: “eslint -c .eslintrc.js ‘**/*.js’; exit 0”.
• Build the apk on two cellphones to test the push notifications, because sometimes in does not work in the simulator.

Mahmoud AlyuDeen what is your Twitter handle ?