I need to implement push notification feature in my application. Exploring some options.
Google recently recommends Firebase platform, but this is not completely a free service. So I thought of using GCM again. Will google stop support for GCM in future?
Yes, it's free. You can use Analytics for advanced messaging - out of the box.
As of April 10, 2018, Google has deprecated GCM. The GCM server and client APIs are deprecated and will be removed as soon as April 11, 2019. Migrate GCM apps to Firebase Cloud Messaging (FCM), which inherits the reliable and scalable GCM infrastructure.
There is no such thing "unlimited" for free. If you will send billions of billions messages a day, you will likely hit the maximum throughput limits and get blocked, or perhaps some sales man will contact you.
Firebase Cloud Messaging is free of charge, just like Google Cloud Messaging was. The Firebase pricing page shows exactly what Firebase features are paid.
https://firebase.google.com/pricing/
This link should help you understand better. If you are implementing push notifications my suggestion would be that maintain your own database for the generated device tokens instead of using topic messaging.
Two reasons why I avoided topic subscription is : 1. There is a limit to the number of topics you can subscribe in free plan 2. Topics are not subscribed immediately
Although the number of active users allowed in parallel connection has a limit but its too much to exceed that easily.
P.S. I use the spark plan by the way