At a sharper level of resolution one sees something very different. Gyges is a shepherd who leaves his sheep to wander into a hole leading under the earth, where he finds both treasure and corpses. In the deepest part of the pit he finds a bronze horse, and inside the horse is a corpse wearing an invisibility ring. Gyges, in other words, leaves off the care and tending to the good of others because he becomes fascinated with the underworld, and with each step he becomes more and more fascinated with death and treasure. At the center of this Hell one finds a Trojan horse, i.e. something that everyone takes as a gift from the gods but which is in reality a curse. Gyges takes the ring, i.e. he betroths himself to the totality of this underworld and in doing so becomes a ghost. He dies in the underworld and brings death back with him.
This is an excellent point, and that the underworld aspect of the story is essential is clear from other things in the Republic. It particularly links the story with the Allegory of the Cave. I've noted before that the underworld has a recurring role in the dialogue. For instance, in the beginning of Book III, one of the passages from Homer that Socrates criticizes is Homer's account of Odysseus talking to Achilles in Hades, in which Achilles says he would rather be the living slave of a poor master than king of the dead; but in the Allegory of the Cave, he quotes exactly this passage: the man who goes out of the Cave would rather be the slave of a poor master in the real world than live as people do in the Cave. The Allegory of the Cave is deliberately flipping the meaning: instead of Achilles saying that he would rather be one of us than live as a hero in the underworld, the Allegory teaches us that it is better to be a Socrates (say) living in the real world than to live in the shadowy underworld like we do. Thus the fact that Gyges enters the underworld is certainly important.
Likewise, it's commonly recognized that ascending and descending are important to the Republic, since they keep recurring (for instance, Socrates descends into the Piraeus to have his discussion with Thrasymachus), and we find here another link between the Allegory of the Cave and the Ring of Gyges. In the Ring of Gyges, Gyges descends into the underworld and then reascends; in the Allegory of the Cave, the freed man ascends out of the Cave and then redescends. These are pretty clearly mirror images: Gyges brings the underworld way to the upper world; the freed man brings the upper world way into the underworld. And James's other point about this is particularly relevant: the freed man redescends into the Cave for the good of others, to bring them real life; Gyges reascends wholly devoted to his own good rather than the good of others, bringing death.
Socrates does this kind of overturning elsewhere, too. In the Gorgias, for instance, Callicles says that Socrates will be dragged to court and, unable to defend himself against orators, will be put to death; in response, Socrates tells a story in which everyone dies and are dragged to a court in which people like Callicles and the orators cannot defend themselves. This pattern of overturning is worth considering throughout the dialogues.