The Martini’s savory second cousin, the Gibson uses a pickled cocktail onion in place of the typical briny olive to add an umami undertone to the classic cocktail. The Gibson is believed to have been created by San Francisco businessman Walter D.K. Gibson in the late 1800s, who thought that eating onions prevented colds, hence the addition of the mini allium.
I personally consider the brine of either olive or onion as part of the flavors of the drink.
What I want to state here was more about the story behind this drink.
Without any documented research only from my notes from mixoligy class at a famous international culinary school.
The story goes Mr. Gibson was a Madison Ave. advertising man who was a tea toddle and he drank water disguised as a cocktail and the onion was an indicator to distinguish his beverage from the rest