If something is good for one person, it should be equally as good for another person; someone who treats another in a certain way should not complain if the same is done to them.

Etymology

From the earlier expression From earlier what's sauce for the goose is sauce for the gander. In 1670s, figuratively using goose/gander for women and men, and literally meaning that the same sauce applies equally well to cooked goose, regardless of sex. Early forms include as deep drinketh the goose as the gander and similar As well for the coowe calfe as for the bull (1549). The expression appears in Dickens when a spy attempting to evade culpability insists,For you cannot sarse the goose and not the gander.