In chapter 8 I need to find a quote that gives an allusion to Cinderella and the importance of the dog leash for George.
I just finished re-reading Chapter 8, and while I see a possible allusion to "Cinderella," I am uncertain whether this actually is. So, I will just offer it and explain why I think it might be. This is in the section in which Gatsby is telling Nick the story of his earlier romance with Daisy.
Gatsby tells Nick, "She vanished into her rich, full life, leaving Gatsby - nothing" (156-7). This is reminiscent of the story of Cinderella because the Prince loses a beautiful woman he has just met, and Gatsby "loses" Daisy. The problem is, of course, that the characters' statuses have been "flipped." In Cinderella, the female is very poor and disappears into the night to go to her place by the fire, tending the ashes, while the Prince is a wealthy young man. In Gatsby's story, the disappearing female is wealthy, a kind of "princess," and the young man who loses her is very poor.
The dog leash, to George, is evidence of his wife's adultery. The couple does not own a dog, the leash has been hidden away, and it is a very expensive leash. All of this tells George that the leash must have been a gift from a wealthy lover, which is consistent with the expensive car that has killed his wife. George has other reasons to believe his wife was not faithful to him, but the leash is a tangible symbol. Whether the leash is meant to be a symbol of Myrtle, who has been kept on an expensive "leash" by her lover, is up to you to decide.