I want to write implement 3rd party server for GCM (Google Cloud Messaging) for android using .Net. The official documentation gives guidelines for using it with servlet-api
If you're not keen on rewriting the whole thing, perhaps IKVM.NET will help? It is a Java virtual machine for .NET and has tools for Java-.NET interoperability as well.
Hope this helps!
The Google Cloud Messaging Client Library has been ported to C#: https://github.com/Redth/GCMSharp
The same developer has produced a library for sending server-side push notifications to iOS, Android, Windows Phone, and Blackberry: https://github.com/Redth/PushSharp