I have found library PushSharp for asp.net MVC 5 and a sample for using GCM.
var config = new GcmConfiguration ("GCM-SENDER-ID", "AUTH-TOKEN", null);
As I understood GCM-SENDER-ID is App ID from goodle console. But what is AUTH-TOKEN? I tried to use api key, but sample throws "wrong authentification" exception. I found some tutorial on the net and somebody have the same quation https://recordnotfound.com/PushSharp-Redth-3794