I recendly swtiched over from Google Cloud Messaging to Firebase Cloud Messaging.
With GCM I had to choose the sandbox option. As described here : https://developers
It is about the setAPNSToken() function. You need to set the FIRInstanceIDAPNSTokenType to Prod while adding the device token. I use swift for that, the code I used is this:
func application(application: UIApplication,
didRegisterForRemoteNotificationsWithDeviceToken deviceToken: NSData)
{
FIRInstanceID.instanceID().setAPNSToken(deviceToken, type: FIRInstanceIDAPNSTokenType.Prod)
}
also if you just want to remove the warning, you may use a production provisioning profile.