Google Music for Mac
Just found this: http://kbhomes.github.io/google-music-mac/
It is a wrap around for Google Play Music that looks like a native app and responds properly to the media keys on the mac keyboard.
Seems to be working fine so far, but Lifehacker think it will be shut down pretty soon for infringement over the Google brand.
Get it while you can, I guess (for free).
P.S. It was on Lifehacker today - I'm not connected with it in any way apart from being a user.