Gorilla glass is not scratch-proof. It's just tempered and treated to make it less susceptible to cracking and shattering.
If you were to look at a magnified cross-section of a scratch in a regular glass screen, it would look like the letter "Y", where the top "v" shape is the gouge on the surface of the glass, and the stem coming down from the from the intersection point is a micro crack that goes straight down into the middle of the glass. You can't really see that micro crack looking at the screen, but it's there. If the glass is compressed, that micro crack can grow all the way through the glass, becoming a crack, and it can lead to more cracks and shatter the screen.
If you look at a magnified cross-section a scratch in gorilla glass, instead of a "Y" shape, you're more likely to see just a "V" shape. The way gorilla glass is created prevents the stem part of the Y shape from forming. So, a scratch is still a gouge in the glass, it just doesn't have (or is resistant to developing) a micro crack going into the glass. This makes gorilla glass screens more resistant to cracks and shattering as a result of compression. Don't confuse compression with impact. There may be an element of compression in an impact, but a fall can still crack/shatter gorilla glass.
Now, as for scratching the glass, all it takes is for something equally hard or harder than glass rub up against the screen. Sand, salt, grit, can scratch it. This stuff is everywhere (beaches, around dirt, pavement, kid's sandboxes, etc), and it can easily wind up in your pocket or whereever you keep your phone. Heck, I've had sand and grit in brand-new pairs of jeans. It can scratch the screen...even gorilla glass. Diamonds are harder than glass and can scratch it (be careful reaching blindly into your purses, ladies).
My employer provided me a mobile phone with gorilla glass, but no screen protector. While on vacation at the in-laws, I put the phone screen-down on the nightstand while it charged overnight. I put it face down so the blinking notification light wouldn't wake me. The next morning I discovered that a few grains of grit that was on the nightstand scratched the screen when I put the phone down or picked it up. It happens.