In the relevant chapter of The Madwoman in the Attic, "The Parables of the Cave," Gilbert and Gubar put forward an explicitly feminist interpretation of Plato's parable of the cave as set out in Republic.
In the original parable, men are chained inside the cave so that they never see each other or anyone else. They can only see the shadows of other people on the walls of the cave, which they mistake for the actual people themselves. Plato argues that this is an allegory for how most people—i.e. not philosophers—see the world around them. Just as the men chained up in the cave mistake shadows for real people, so too do most people confuse the world of objects with ultimate reality, which lies only in the Forms—those abstract, unchanging ideal concepts from which true knowledge is derived.
As one might expect, Gilbert and Gubar offer up a radically different take on Plato's parable. They present the cave as "a female place, a womb-shaped enclosure," the site of a uniquely feminine artistic heritage which goes all the way back to the mother-goddess myth of the Cumaean Sybil. Far from being a place of shadows and ignorance as it is in Plato, the cave represents for Gilbert and Gubar a repository of female memory, a place where the female artist "redefines and recovers the lost Atlantis of her literary heritage".
For Gilbert and Gubar, then, women's art represents a return to the cave, a remembering of what has been forgotten, a gathering in and painstaking reconstruction of all the pieces of a shattered, neglected tradition that women can recognize as their own.