What are some alternatives?
When comparing Google Cloud Functions and Pusher, you can also consider the following products
Google App Engine - A powerful platform to build web and mobile apps that scale automatically.
Firebase - Firebase is a cloud service designed to power real-time, collaborative applications for mobile and web.
Dokku - Docker powered mini-Heroku in around 100 lines of Bash
PubNub - PubNub is a real-time messaging system for web and mobile apps that can handle API for all platforms and push messages to any device anywhere in the world in a fraction of a second without having to worry about proxies, firewalls or mobile drop-offs.
Azure Web Apps - Create and deploy mission critical Web apps that scale with your business
Socket.io - Realtime application framework (Node.JS server)