Gorilla Glass is not indestructible. It is only scratch-RESISTANT. Even if the S3 had Gorilla Glass (US, overseas, etc), you should still handle your phone with care. Thinking Gorilla Glass wil protect your screen from everything is almost as bad as thinking the Droid Razr line of phones are bullet-proof due to having Kevlar backing.... almost lol
Seriously though, it's your choice to protect your phone. I've never had a cracked screen before, but almost all of the cell phones I've used have had a case and a screen protector. (touch screens. didn't need or make them for those old Nokia brick phones lol). Better to be safe now than pissed off later.