Just in case someone else comes accross this q/a: the situation has changed. With Xamarin.Forms 3.0, Gtk# is supported (as preview, at this moment). Therefore, full Linux GUI support is enabled.
So, Xamarin now covers:
- Android
- iOS
- UWP apps
- WPF apps
- Linux Gtk desktop applications
- Mac OS
- Watch OS
- tv OS
- Tizen
The only thing left to wish for: JS/HTML5 Web App target platform, as part of Xamarin 🙂