I believe the passage you are referring to found in Chapter 1 and involves Nick's exchange with Tom Buchanan the evening Nick goes to visit Daisy and Tom for the first time in East Egg. When Tom asks Nick what he is doing, meaning doing for a living, it leads to the following exchange:
"I'm a bond man."
I told him.
"Never heard of them," [Tom] remarked decisively.
This annoyed me.
"You will [know them]," I answered shortly. "You will if you stay in the East." (Ch. 1)
What Nick is implying here through his annoyance is that Nick will soon put the bond company he works for on the map and that Tom certainly will hear of the company if Tom remains on the East coast. The reason why Nick feels compelled to imply something so boastful is because he feels annoyed by Tom. For one thing, Tom saying that he has never heard of Nick's company can come off as sounding a bit insulting, especially from a man as powerful and financially endowed as Tom. Hence, Nick decides to reply in a way that will trump Tom's insult by being boastful about his abilities as a bond broker. Another reason why he feels annoyed at Tom is because he feels that Tom is not caring well for Nick's cousin Daisy. Nick knows that after their marriage, Tom and Daisy became a bit restless. Nick learned that they spent some time in Paris, for no known reason, until they finally wound up in East Egg. While Nick doesn't know how Daisy feels about Tom's restlessness, Nick can see that Tom's restlessness is stemming from the fact that Tom is wishing for his old college glory days when he was a football champion, as Nick states:
I had no sight into Daisy's heart but I felt that Tom would drift on forever seeking a little wistfully for the dramatic turbulence of some irrecoverable football game. (Ch. 1)
Hence, since Nick sees Tom as a restless soul and also one whose greatest achievements are over but who is regardless selfishly, indulgently rich due to inheritance, Nick also sees Tom as a man who cannot care well for his cousin Daisy, and it is essentially Tom's lack of ability to care that is truly annoying Nick rather than just Tom's subtle insult.
Hence, Nick's curt reply in answer to Tom's insult actually underscores a central theme in the book concerning the immoral behavior and weak characters of the absurdly rich.
In Chapter 1 when Nick says "You will hear of them" it is because he becomes annoyed at Tom's responses, like the previous comment was explaining.
For instance: "His comment annoyed me. I responded, You will hear about them soon!" It clearly states here that he was annoyed hence the response somewhat illustrating sarcasm at Tom.