Before we talk how to generate your GCM Sender ID and GCM Server Key, let us take a quick shot at answering these questions. What Is GCM Service? Google Cloud Messaging is a free service that enables developers to send messages between GCM servers and client apps. This includes downstream messages from GCM servers to client's app, and upstream messages from client apps to GCM servers. Starting May, Google Cloud Messaging Platform is being replaced by Firebase Cloud Messaging.