well, I had this complain about scratches on my screen while having gorilla glass. And my co worker (was working on product side of Sony) said that gorilla glass's only aim is keeping the screen without getting broken on major fails, being more resistant to impacts.
I used the phone without screen protector, didn't even put it to same pocket with keys or coins (duh) and it had scratches on it, it was easily noticed if you turn it like 45 degrees.
And a month later I dropped the phone on asphalt on left top corner, screen was safe but there was big bump (screen looks bumped up on that corner). So that was the real purpose of gorilla glass.
Only way to prevent scratches on screen is using screen protector.