Despite numerous videos demonstrating that it scratches from exposure to softer materials than the front glass easily withstands, Google did state that the back pane is Gorilla Glass 4 just like the front...
Most people with concerns over it are putting their phone in a case or getting a skin to cover either the full back of the phone, or at least the glass (Dbrands is good stuff, albeit a bit slow to ship, and has both options available).