Gorilla Glass is a brand name/model (formula) of reinforced glass. It's what everyone thinks of when they hear a phone has scratch-resistant glass, much like people think 'frisbee' when referring to what others simply call a 'flying disc.'
I don't know Corning's product line, but I suspect that Gorilla Glass is simply their premier product, and that the Corning Fit Glass is a more basic model with lower price but sacrificing something in terms of scratch resistance/thickness/flexibility.
Gorilla Glass is almost ridiculously scratch-resistant, but after a couple of years carrying my GG-equipped phone in my pocket with keys and change, I did suffer two small scratches. The N7 will be taking less daily abuse by nature, but it is still worth considering protection unless you are exceedingly careful and would not be bothered by slight scratches should they occur.
As to how a finger could cause a scratch on a harder surface, it would be by unintentionally sliding a particle or mote of debris of some form across the glass that was hard enough to etch it.