As of September 2016, you can no longer create Google Cloud messaging API keys.
1. Click on Settings, Cloud Messaging, and the press "Add Server Key" to create a new server API key:
2. Copy the new server API key from the Firebase settings panel to your clipboard
3. Add the API key to your Ably Notification app dashboard
Then paste in the newly generated server API key into the text box beneath "Setting up Google/Firebase Cloud Messaging" and click "Save GCM/FCM settings". Once saved, the page should look something like: