I have experience in programming for Linux, Android and iOS. Me and my team strongly believe that the best approach for GMR is to have a single implementation that could work for Linux, Mac, iOS, Android and Windows Phone. We recommend using QT ( www.qt.io ) as it is very robust, widely supported and multiplatform. QT has an open source SignalR client: https://github.com/p3root/signalr-qt

To allow for the per platform look and feel, it is easy to design different interfaces for multiple platforms (using QML: http://doc.qt.io/qt-5/qtqml-index.html) while keeping the core implementation the same.

I personally consider that mobile apps with APNS and Google Cloud Messaging notifications are essential. I have used node-pushserver for that ( https://www.npmjs.com/package/node-pushserver ) but I'm not sure how comfortable you'd be to integrate with the server (GMR runs on Azure, right?). Even if you'd rather implement it in C# there might be some libraries for that, I do not recommend implementing it yourself because mapping device IDs to users can be more complicated than it looks.