I've no idea of FCM and GCM and right now I have to receive notifications sent from server. Is it compulsory for me to add my project to firebase? Thanks in advance!
Firebase cloud messaging is just a way to send push notifications to your application(there are other alternatives).
As for adding your project, you will just be giving few information like your App name. This will generate you a secret key to use to access the FCM service. There are several tutorials available on the net and its free(they won't bill you extra if it exceeds your usage limit).
Have fun exploring FCM
Leaving you a tutorial link: http://stacktips.com/tutorials/android/how-to-integrate-firebase-cloud-messaging-for-android