We are trying to add our GCM API key to the azure notification hub and we are receiving the following error:
SubCode=40000. Failed to validate credentials with GCM. Error is The remote server returned an error: (401) Unauthorized...TrackingId:410c0e33-0c38-1823-8a62-2519627c76a2_M1_G19,TimeStamp:9/19/2014 10:08:32 AM
We have setup our GCM API key to allow any IP address and we have also tried registering from code like below but with the same error:
NamespaceManager mgr = NamespaceManager.CreateFromConnectionString(conn);
var hub = mgr.GetNotificationHub("<hubname>");
hub.GcmCredential = new GcmCredential("<apikey>");
mgr.UpdateNotificationHub(hub);
Are we missing something quite obvious here?
SubCode=40000. Failed to validate credentials with GCM.
This will probably fix this error:
Under the SERVICES tab in the APIs console for Google devs:
- enable Google cloud messaging for android
I recently had this problem and what I found was that I needed to go to the new firebase console, import my existing google project.
Once I had done that I could go to the settings cog in the top left->project settings. Then click on the cloud messaging tab then get the new server key. Once I entered that I was good to go in the notification hub.
CrusherJoe is 100% correct but if some one need more clear picture here I have attached the screenshot. just go to the link and enable it. 
Some users may not have verified or registered domain.
With the help of webmaster Tools u can verify and register your domain.
Go to Google Dev Console -> your project ->API's and Auth -> Push -> Click on Learn more for webmaster
来源：https://stackoverflow.com/questions/25931532/adding-google-gcm-api-key-azure-notification-hub-error