The Gibson is a mixed drink made with gin and vermouth, and often garnished with a pickled onion. The oldest published recipe for the Gibson is found in the 1908 book, The World's Drinks and How to Mix Them by William Boothby.
"Note – No bitters should ever be used in making this drink, but an olive is sometimes added."Since the earliest known definition of the word cocktail as a type of drink in The Balance and Columbian Repository from 1806 mentions that the type of drink is also called a "bittered sling" one could say that, by those traditional standards, the Gibson is a sling rather than a true cocktail.
Other pre-prohibition recipes for the Gibson exist. They all omit bitters and none of them garnishes with an onion. Some garnish with citrus twists. Others use no garniture at all. No known recipe for the Gibson garnishes with an onion before 1922. Some sources persist in using other garniture than the onion into the 1930s and beyond, but still none use bitters. According to pre-prohibition sources, the 'classic' Martini of today without any bitters is actually the Gibson. However, modern terminology favors reserving the Gibson name for the same drink only when garnished with an onion.
1 1/2 oz gin
3/4 oz vermouth
2 cocktail onions
Stir gin and vermouth over ice cubes in a mixing glass. Strain into a cocktail glass. Add the cocktail onions and serve.