Keeping the deprecated status of the GCM effective from 10 April, 2019 Informvisitors has upgraded all the users from GCM to FCM. To make use of the changes made by us users are required to update their existing manifest.json file in their root folder.
Note: Instead of modifying the whole manifest.json file users can just update gcm-sender-id value from 539434688778 to 103953800507.
That's all. If you have completed the above steps, by now you will be successfully migrated to FCM.