random music appeared on google music (on device)

Is it really saved on your N7 or is it streamed from the web. If anything, clean the cache.

 

Same here ... A bunch of music I don't like or want except for maybe 2 or 3 songs that sounded a little interesting, they get loaded all by themselves and just appear in your music list so I just deleted them.

 

You got some free music with the purchase of the nexus 7.

Just delete it if you don't want it.

 

They are indeed files from a game or app that haven't been properly hidden. Search your internal storage for the folders called casino and classic. When you find them, don't delete them. Instead, create a blank file (text file is probably easiest) and remove the extension and rename it .nomedia
If you place that in both folders it should exclude any media in them from the Android library but they'd still be usable by whatever app they're from.
Might take some time before your library is rebuilt, restarting might trigger a media rescan.