Fill a mixing glass with ice cubes. Add all ingredients. Stir and strain into a chilled cocktail glass. Garnish with a cocktail onion.
There is a story about Gibson -- that it's named after a tee-totaling diplomat named Gibson. As he'd often go to cocktail parties, he'd ask the staff to fill his glass with water instead of a cocktail, and garnish it with an onion to be able to distinguish it from the rest of the guests' drinks. True or false? Don't know, but it's a darn nice drink.