Can LG Optimus G get scratched on Gorrila glass and does it requires additional case and screen protector to keep device unharmed (without scratches and visual defects) ?
The short answer is yes, it can be damaged. Gorilla glass is not a guarantee against scratches -- it sure helps when you put your phone in a pocket with keys, coins and stuff, but a tiny grain of sand can still leave a scratch on it (not saying it necessarily will, but it certainly can). The back cover is relatively easily scratched and scuffed, especially in the corners (I have some visible wear marks on it, but don't have a second camera to take a photo right now). The bezel on this phone can also easily get nicked, as the plastic is pretty soft.
So, I would definitely recommend some protection, if you care about the appearance of your phone. I personally decided against using screen protectors quite a while ago, for number of reasons (I still use it on my tablet, but not on phones), and I just try to be careful with the screen. So far, so good. But a good case for me is a must. It also makes the OG easier and safer to handle, as the naked phone is quite thin and slippery. There is a lot of decent cases available for our phone. You can also take a look in the Accessories forum for some first-hand reviews and comparisons.