I handle over a dozen smart phones per day so I'll lay out what I consider a well balanced diet of info.
I've seen gorilla glass scratched LIKE CRAZY to the point the phone's touch screen starts malfunctioning. It had never been out of the ladies purse beyond use and phone calls. She wore one diamond ring, and had a set of very nice diamond earrings.
Gorilla glass is not invincible.
You can run car keys over it all day with no issue, but with diamond jewelery, hand tools (like screw drivers, chisels, pocket knives, etc. anything with hardened steel) rock chips, sand, other stuff like that, will scratch the hell out of it.
Weigh your usage and exposure to these items when considering a screen protector.