While not a full GU story in its own right, I should really mention Outside Over There by Maurice Sendak. Primarily, because it was the initial inspiration for the movie Labyrinth, which in turn inspired the whole Girls Underground concept.
In this beautifully-illustrated picture book, Ida must care for her baby sister while her father is away at sea, and her mother is equally absent in her own way, waiting for him. Goblins come and steal the child, and so Ida sets off in pursuit “out her window into outside over there.” She finds the caves of the goblins, blows her horn and reveals them to all be babies too (which vaguely echoes the “revealing a fraud in the adversary” theme). Then she finds her sister among them and takes her back home.
There is no individual adversary, no companions, and the whole thing is really too brief to be a GU story, but the basic elements are there, and it sparked the template upon which the whole archetype was originally based, so it feels right to include it here.