Do I need to migrate GCM to FCM on client side?

Existing installs of client-side apps receiving notifications will continue to work for the foreseeable future. You don't need to get new tokens for all users. The existing tokens from GCM will continue to work

But from mid-April:

1. You won't be able to compile code using the old client-side APIs on a newer SDK/Google Play Services anymore
2. You won't be able to call the GCM REST API to send messages anymore
3. New installations of your existing GCM-using app may not be able to get a token.

To compile against the new SDK/Google Play Services, you will need to make changes to your client app to migrate to Firebase Cloud Messaging from GCM. This involves changes to the build.gradle, to AndroidManifest.xml and to your code, as described in Migrate a GCM Client App for Android to Firebase Cloud Messaging.