What accomplishment of Meyer Wolfsheim's does Gatsby describe to Nick? How does Nick react?
Concerning your question about The Great Gatsby, I don't know if "accomplishment" is the best word to use, but Gatsby tells Nick about Wolfshiem's fixing of the 1919 World Series.
Here's Nick's reaction:
The idea staggered me. I remembered of course that the World's Series had been fixed in 1919 but if I had thought of it at all I would have thought of it as a thing that merely happened, the end of some inevitable chain. It never occurred to me that one man could start to play with the faith of fifty million people--with the single-mindedness of a burglar blowing a safe. (78)
Gatsby then explains that Wolfshiem simply saw the opportunity, and took it. When Nick asks why Wolfshiem is not in jail, Gatsby tells him that Wolfshiem is a smart man--they can't get him.
Nick then spots Tom in the room, and the conversation ends. Nick is staggered and, if you will, flabbergasted. The idea that a single man could be behind something so big never occurred to him.