Actually, I don't think the iPhone 4 and iPad have Gorilla Glass. Here's one independent source stating as much, and I don't think Apple has ever directly claimed they use Gorilla Glass.
Additionally, the Apple glass can be scratched in normal use.
I think this is an urban myth that the iPhone and iPad use Gorilla Glass.
http://www.tested.com/news/whats-so-special-about-iphone-4s-aluminosilicate-glass/429/
Of course, the new iPhone is promising a similar, indestructible style of screen — one that’s curiously similar to Gorilla glass. Apple promises the screen to be “20 times stiffer and 30 times harder than plastic” and also incredibly scratch resistant as well. This is all due to a panel of aluminosilicate glass, the same sort of material used by Corning to manufacturer their own device screens. In fact, the similarities are so consistent that some have questioned whether Gorilla glass is behind Apple’s new phone. We’ve reached out to the company, but have yet to receive a reply. — something that’s neither been confirmed nor denied by the company.
The iPhone4 is Corning Gorilla Glass... both the screen and the back glass. Apple and Corning have both stated that it is. . . Steve Jobs stated it as a fact in the introduction of the iPhone4. The glass is made by several manufacturers under license from Corning.
As for your link to the scratched iPhone: Again, you find the stuff from the fringes and post it as fact. I have many friend with iPhone4s and none of them, including myself, have succeeded in scratching our phones.