"it is what preyed on Gatsby, what foul dust floated in the wake of his dreams that temporarily closed out my interest in the abortive sorrows and the short-winded elations of men" (Fitzgerald 2). Clearly, in the most literal sense, Gatsby didn't turn out alright in the end. So what does Nick mean by this, and what "preyed" on him?
Nick admits that he "disapproved of him from beginning to end", but he still found Gatsby such a polarizing character that he had to remain a "friend" to see what transpires. Nick's role as a pure observer in the story allowed him to understand Gatsby's motivations to every end. He saw Gatsby's need to flaunt and show his wealth as a way to impress Daisy. His drive to be a "Trimalchio"-esque character, as Nick pointed out, was what preyed on him. It was his sole purpose to have Daisy back, as the "foul dust" that "floated in the wake of his dreams", was the memories of the past that he never got over. The events past took over his decisions.
This and his death suggests that he didn't turn out alright, but remember that this is from Nick's point of view. Nick seems to be a cynical and neutral narrator, but he still holds his own opinions. He did not particularly like Gatsby, especially as the story went on, but he saw why he was the way he was. Maybe his demise was the good ending, and arguably he got everything he wanted, even if it was only for such a little time. The past took over his motivations, and set his one singular goal in life. He achieved that goal, and so maybe he did turn out alright