A Gnawr hides in a hard, protective shell. Their necks are usually tucked in for defense but when fully stretched they can reach nearly two feet in length. Their bite is powerful, strong enough to puncture krogan armour and cut through bone. A full grown Gnawr can take down a varren. Unsurprisingly, they have no natural predators, with the possible exception of the kakliosaur. Everything else seems to give them a wide berth. They are omnivorous and can feed on shrubbery or prey on other animals. During the day they burrow into the sand to protect themselves from the heat, migrating at night. They have paddle-like appendages that help them move and dig through the sand.
Gnawr aren't found around populated areas as noise tends to disturb them. Colonies bury themselves in the sand out in the wastes. They're not a common meal for krogan since they're difficult to hunt, even with a hunting varren (and many don't want to injure their prized warbeasts for something so trivial). Their shells are usually collected when stumbling upon a dead one. Female camps use them as stew bowls, or toys for children. The young krogan take turns sitting inside an empty shell while another pushes them down sand dunes.
Clan Raik has a story about a grouchy Gnawr who made friends with a pyjak. The pyjak gave it handfuls of water to drink and when it was well, it ate the pyjak. The moral is obvious: don't trust a Gnawr.
Shadow Pyjak has noted that when she was stationed in the DMZ some soldiers had to go planetside on Tuchanka to investigate the actions of a small krogan clan. They stumbled upon some rocks; one soldier was frustrated and kicked at one. It was a Gnawr and it responded by biting half his foot off.