Which Characters in The Great Gatsby fit the description of stock, dynamic, and round?
Concerning the types of characters in The Great Gatsby, I think that maybe no editors have answered your question yet, because two of the three terms you ask about don't really apply to the novel. I didn't answer it the first time I saw it, either--I was hoping someone else might have a complete answer for you, because I only have a partial one.
In other words, I'm not sure any stock or dynamic characters are really featured in the novel. Characters are well-developed (the "round" part of your question), but no often-used, stereotypical stock characters really exist, and I don't know that anyone really changes much (dynamic). Wilson is the possible exception, with his murder of Gatsby and suicide, but he certainly isn't a major character.
Even Gatsby, the most well-developed and definitely round character, doesn't change. That's what makes him Gatsby. His love is an idealistic, all-encompassing love, and he never gives up on his chance to get Daisy back and recapture his past. He's still waiting for a call from Daisy on the last morning of his life. He never comes to a realization or epiphany of any kind.
Gatsby is round, though. He is a mystery man who is shy, he's almost always polite yet seemingly uncaring about anything that doesn't concern his quest to get Daisy back, has a library full of books but doesn't read them, loves, perhaps, like everyone wishes he/she could love, was born fairly poor but becomes extremely wealthy, makes his money illegally, at least in part, and yet is an obsessive idealist.
Daisy, too, is a round character. She's a victim of her patriarchal society, cynical, difficult to "read" or interpret, manipulative, beautiful and knows how to use her beauty to get what she wants, and is at least somewhat amoral. Yet, though she despises Tom and may actually love Gatsby, she refuses to say something that isn't true--that she never loved Tom, even when they were newly married. She rejects Gatsby, in the end, because she won't say what isn't true, because Gatsby asks too much: he insists that his dream is true, that Daisy's been pining for him these five years, and never really loved Tom. And Daisy won't say it.
Gatsby and Daisy are certainly round characters, but, again, I just don't think the other two types of characters you ask about apply to this novel.
Regarding stock characters, which are stereotypes, in The Great Gatsby, Meyer Wolfscheim can qualify as one. For, he fits the cultural type of the Jewish Mafia that was in power in New York in the late nineteenth century and early twentieth century. Meyer has the typical hook nose of this stereotype, he speaks in a Lower East Side accent, and he is tasteless in his clothing and, of course, his cuff links made from human molars--possibly someone that he had "snuffed out."
Jewish-American gangsters were involved in such activities as bootlegging and racketeering. These gangs controlled the Lower East Side and Brownsville in New York City. These tough guys extorted, exploited, and murdered other members of their community for profit. In one of his conversations, Wolfscheim speaks of a fellow Jewish gangster,Rosy Rosenthal, who has been murdered; also, he discusses his involvement in the fixing of the World Series in which the White Sox played (Fitzgerald modeled Wolfscheim on the gangster Arnold Rothstein), as well as offering Nick a job in his "business."
I agree that Wolfshiem is a stereotypical and therefore stock character, and that Daisy and Gatsby are most definitely round and complex. The dynamic character is Nick. It is interesting to watch Nick's progression through the novel, as he moves from a character fascinated and intrigued by the wealth of people like the Buchanans and Gatsby to disenchantment and a desire to return to the West.
Nick turns 30 in the novel, and in many ways matures. He wants the world to stand at "moral attention." He knows that Gatsby is better than the "damned lot of them." He is realizes that there is more to life than empty materialism, trying to regain youth, and living in the past. He begins the novel seeking his own fortune in the bond business. He ends the novel returning home to become grounded once more. Nick sees through the emptiness of life in the East. He needs more. His realizations about himself, the American dream, and those whom he formerly admired make him a dynamic character.