gMusic for Windows 8 and Windows RT has been updated again. I use this all the time for listening to Google Music on my Surface RT and this update improves the integration. The latest updated adds support for adding songs not in your library to playlists (if you have a Google Music subscription) and there are bug fixes for the snapped view and buttons being disabled.
Google have said they are not developing for Windows RT but at least we have 3rd party developers coming up with great solutions.
The app is free from the Windows Store.
[+] Add support for songs in user playlists not from user library (all access mode, before gMusicW ignored them).
[b] In snapped view rating not always was updated for current song.
[b] Player keeps next/previous buttons disabled after user adds songs to queue.
[b] Fix issues when gMusicW can stop to auto sync library.
[b] Sometimes gMusicW does not respond on ‘Keep local’ button for selected playlists.