A gawk block is a plastic rectangular tool with a rectangular opening in it on one side. It is usually used in Marching Bands by striking over the side with the opening with a drum stick to produce a loud and piercing clicking noise, called a gawk, to keep time for the band by the drill instructor or drum major.
It is very irritating when the person keeping a beat on the gawk block passes very close to you, because it hurts your ears.