Have you verified this to work on the Gnex? It does not work on mine. I'm running a 4.2.1 rom however so has this been verified to work with a stock, unlocked Gnex and if so are we talking ICS only, JB or both?
When I run it, I get a backup folder and the file created ok, but the file size is 0kb and the "backup" happens in a few seconds so it's pretty obvious it's going to fail. Could be because I'm rooted but I'd think this would work on any setup unless it's looking for an exact, stock file structure. Then the fail might make sense.