Guts glanced around with his one eye(since the other one was stabbed out by an apostle during the Great Eclipse). Most people would think that he's just keeping the other eye closed, but he wasn't. He only had one eye, along with only having one arm, since he personally cut off the other one. Well, more along the lines of stabbed himself in the arm with a broken sword until it came off. In its place, there was a metal arm, which he had some control over, but not a lot.