Native Pushs - What am I missing?

Regarding to http://docs.ionic.io/docs/push-android-setup ...

Calling https://console.developers.google.com/home/dashboard?project=MyApp I see the projectId and the projectNumber of my registered project (I although have got a google-api-key by registering google push api on that project). So now I have three Ids: The google-api-key, the projectId and the projectNumber.

I used the following console commands:
ionic push --google-api-key google-api-key
ionic config set gcm_key projectNumber

Was this correct? Or should I have set for gcm_key something different than the projectNumber?

Another question: What is senderId? Do I have to set it?

Ionic.Push({
    "pluginConfig": {
        "android": {
            "senderId": ???
        }
    }
});