In part 2 of Dark Carnival you'll come across a well-lit shooting range with carnival mascot Moustachio emblazoned on it. Also note the yard gnome in the box -- your prize for getting 750 points. Get this high score by activating the game (via the red button) and shooting everyone but the blue peanut (you'll lose points if you hit this target!). Hit the yellow and green Moustachio target for extra points. The gnome (which you may recognize from Half-Life 2 Episode 2) will be released if you get a high score and so will scores of infected, so beware!.

Carry the gnome all the way to the end of the scenario (board the chopper) to unlock the Guardin' Gnome Achievement. You can drop Gnome Chompsky and pick it back up. You can also drop it in a safe room and it will be there after the break. Finally, you can use the gnome to repel zombies as you would with a weapon.