Yes, but both are slang units of distance and diameter. The eyes of typical gnats tend to have diameters similar in size to a hair’s breadth—roughly 100–150 micrometers. An item would have to be very short in order to be gnat’s eye in length! A hair’s breadth is an informal unit of distance: it is used to denote a measurement of approximately 70 to 100 micrometers in diameter, or 0.1 millimeter, which is similar in thickness to real human hair.