The story
From earlier what's sauce for the goose is sauce for the gander (1670s). Other early forms include “as deep drinketh the goose as the gander” (1562).
Etymology from Wiktionary, CC BY-SA 4.0.
what's good for the goose is good for the gander
Today it means: Literally, what is good for a female goose is equally good for a male goose (gander); or, what is good for a woman should be equally good for a man. Full definition →