It is clear to me that with a subscription to Google Play Music you can only play "your" music (either online or offline) with Google Play Music player (see Google music: offline listening after subscription expires?). De facto, music is not actually yours: you just own the right to listen to it.
What is not clear to me, and I have not been able to find an answer for, is: what if I actually buy a track on Google Play Store? Can I move the MP3 to any "dumb" MP3 player and listen at it there? In other words: is it DRM-free?