This repo showcases how to send a webhook to a Google Hangouts Chat room using Apex and Process Builder.
Read more about it in this blog post.
And don't forget to check out the other repos:
You need to have an active G Suite organization (sign-up here as a Salesforce customer if you don't have one).
The Chat documentation explains in detail how to setup Hangouts Chat bots.
Deploy the app to Heroku using the deploy button.
For licensing see the included license file.