OK, heres the break down:
Glass is much much harder than plastic, when glass is used it is often labelled as "scratch resistant" (Different types of glass vary in their hardness, Mineral > Soda for example), what they mean is "more scratch resistant than plastic".
The primary function of gorilla glass over normal glass is not scratch resistance, but reduced crack propagation.
As i mentioned glass is hard, but it is also brittle, we all know it shatters very easily. It shatters because when sufficient force is applied to the glass (from an impact or fall for eg) microscopic primary force fractures occur within the glass, because the glass is hard and brittle these cracks propagate throughout the whole pane, resulting in the spider web fractures you are used to seeing.
Gorilla glass is designed such that those microscopic cracks do not propagate, the screen is still damaged, but the damage is tiny and localized.