Lee's selection of The Gray Ghost was significant for a variety of reasons.
First of all, the title alone is suggestive of Boo Radley. When Scout finally meets Boo, she notices how white he is. They have been calling him a ghost name for their whole understanding of his existence. They told stories about him as if he was a phantom or ghost.
Second, the character in the story named Stoner's boy had never really done anything wrong. People had just misjudged him. This is important because it was the very thing that people had done to Boo Radley.
Finally, this character actually turned out to be a hero just like Boo was for Scout and Jem. Although Scout doesn't draw the connections for the reader because she is falling asleep while talking with Atticus, she certainly reveals information that leaves readers the opportunity to draw conclusions in Chapter 31 after she had walked Boo home.