This post contains spoilers for The Great Gatsby.
I saw The Great Gatsby movie a few months ago when it came out, and I was annoyed at some of the parts that didn't make it into the movie. At the time, I didn't really know why I was so annoyed, but I think I figured it out this morning.
I really liked the part in the book after Gatsby's death when his father comes. Specifically how Nick constantly and erroneously calls him "Mr. Gatsby" instead of "Mr. Gatz." I think Gatsby's changing his name from James Gatz to Jay Gatsby is a really significant point that the movie should have included.* It simultaneously changes and hides who he is, and Gatsby's appearance and reputation are huge aspects of the story.
He changes his first name from "James" to "Jay," shortening it. In a way, by doing this Gatsby rejects his past and the poverty that it includes. He wants to be this rich, impressive guy in order to win Daisy's love, and completely switching over to opulence and wealth is one way to do that. He changes his last name from "Gatz" to "Gatsby," embellishing it. Again, this functions as a way to appear elite. "Gatz" sounds normal, but "Gatsby" sort of makes you pay attention to it. (Though that could be just because it's been coupled with "great" for so long....)
In any case, Gatsby's changing his name is just an-other example of how he changes himself. He rejects the common and strives for the extravagant in his attempt to impress Daisy. In the novel, after Gatsby's death, the reader finds out that he wasn't really Jay Gatsby, he was just James Gatz. That heightened persona is totally deflated, showing Gatsby for who he really is. But the way it's done in the movie, Jay Gatsby just disappears into death, and his true identity is never really revealed because Nick never meets Mr. Gatz. I think it's a more tragic ending to have the self you tried to hide exposed than it is to just be ignored.
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*I saw the movie once three months ago, so it may have included this and I've just forgotten about it. In any case, I still feel that Nick's meeting Mr. Gatz illustrates Gatsby's change in a way that no-thing else does.