We've been playing with the GNSDK for the last few days. We've been able to make it work with the Android and iOS platform. But when we tried to do with Windows 8 and Windows Phone 8 we found it to be not supporting any CPU, the Windows Phone SDK only supports x86 architecture and Windows RT supports only x64. If we're able to have both architectures in both the platforms, we will be able to reach a much wider array of users.
Awaiting a reply.