"A Gordon LaChance halfway along in the process of losing the shine." This sentence is from the novel named The Body written by Stephen King. What is symbolized by "the shine," and what is the meaning of this sentence?

Gordon LaChance is the protagonist of King's novella The Body. In this line, he is referring to the version of himself that he feels he can still see "behind the lines of print" of the very first story he ever wrote and was proud of. While he knows that this first "whole" story was not very good, being altogether too melodramatic and "undergraduate," it is important to Gordon because it presents him with a view of himself when he was younger, when he was "halfway along in the process of losing the shine."
"The shine" on an object is something that indicates that it is new. The narrator, the current Gordon, sets himself at one end of a scale, with the other end marked by the very young Gordon LaChance, who went along with his friends to see the dead body of Ray Brewer. It is this young Gordon who is the subject of the current story. The Gordon LaChance who wrote the earlier story, the "undergraduate" Gordon, marks the halfway point between the two Gordons.
The young Gordon, we understand, still has the "shine" about him. This indicates that he is young and naive: eager to experience life but not very well informed about what life is really like. Meanwhile, the older Gordon has lost his shine; he is cynical and no longer possesses the earnestness of youth. The Gordon "halfway along in the process" therefore falls somewhere between the two: no longer as naive and eager as his younger counterpart but not yet fully aware of all that life really is.