Gorilla Glass is the DOW brand name for simple reinforced glass. The GN has reinforced glass, just not the Dow brand. It's still highly scratch resistant.
Galaxy Nexus - Scratch Test - YouTube
Seriously, this test/video is worthless. Key metal will not scratch glass, ever. period. "Gorilla glass" or not. What DOES scratch glass is other glass (sand, silt, "dust"- it is all silica or contains silica), and things that actually are harder than glass (check out Moh's scale
here- Not the only scale, but most popular for rating mineral hardness). Key metal is a mixture of brass, nickel, and steel (mostly), and so is not any harder than about 4 Mohs. Silica (Quartz, glass, etc.) ranges from 6.5 to ~7.2 ("fortified"), well beyond the range of anything but the hardest of hardened steel. So anything 6.5+ Mohs could be a good candidate for scratching risk.
This overview (while partially incorrect) is a much better display than the key test. Why? because it shows that the hardened steel (harder than the glass) does scratch the screen and that these screens are not invulnerable.
This can occur when silica grains (whether sand sized or silt sized) from the air collect either in your pocket, or on the surface of something else (such as a key/knife/finger, whatever) which in turn comes into contact with and drags across the surface of the screen. These can even be "
nano-particles" that carry an electrostatic charge and so are more apt to cling to conductive materials, creating in effect a silica-plated surface much the same way that diamond-encrusted saw blades are metal blades with a diamond grit encrusted on it.
The reason some get scratched while some haven't (yet) is the force dragging the "grit" across the screen has to not only be lateral (parallel to the screen, i.e. moving it from side-to-side), but also proximal (perpendicular or "pushed into" the screen). So let's take the example of someone who just keeps the phone in an empty pocket. It brushes up against the lining of the pocket according to the movement of the body, with some actions perfectly mimicking the sufficient lateral force such as stretching the pant sides as one sits down or stands up or by removing the phone from the pocket, but this likely does not provide enough of a proximal force so it results in no scratches. But if a variable changes in that equation (more constrictive pocket due to the addition of other things into it, wearing of perhaps too tight of pants, brushing up against something on that side- provided the screen is facing outwards, etc.), and "grit" is introduced from the air or beach/desert, one can see the increasing and increasing risk.
Unless a pocket can be vacuum-sealed, scratching the screen is less a matter of what or why, but rather a statistical when. The true issue of course, is whether you (as the device owner) want to play the numbers game or not, and (pursuant to this thread's topic) will you be sad (or on a new device already) when your number is called?