There is a build for Windows 8.1 but it was not build as a universal PCL. Contacted support but they said to ask here, to get a response from the developers.
NOTE: by universal I mean, Windows Store universal, that work on WP8.1 and W8.1
The latest GNSDK for Mobile release should support WP8.1. Please download the one with platform "Windows Phone 8."
Gracenote SDK is a native library. It is not an application. Just like any other third party library (like sqlite - check https://sqlite.org/download.html), it porvideds winrt and windows phone libs. How do you build the library to run on phone and winrt both ? Can you post link from MSDN on how to do it ?. I dont think you can do this for libraries which has native component/s to it.
Gracenote window phone libs are built using visual studio 2012 and should work for both windows phone 8.1 and windows phone 8.0.