In F. Scott Fitzgerald's novel The Great Gatsby, we are certainly given a couple of clues telling us that Jay Gatsby, as well as all of New York, is well aware of Tom Buchanan's affair with Myrtle Wilson.
One clue we have to tell us that Gatsby is indeed aware of Tom's affair with Myrtle is found in the first chapter. Here, Nick spends an evening with his cousin Daisy, her husband Tom, and her friend Miss Jordan Baker. During dinner, Tom is called to the telephone, and Daisy follows him; the sound of their arguing can be heard in the background. When Nick tries to make conversation with Miss Baker, she shushes him, saying she wants to "hear what happens." When Nick responds blankly, Miss Baker asks, "You mean to say you don't know? ... I thought everybody knew," referring to Tom's mistress. Since gossip has a tendency to get around on both East Egg and West Egg, we can assume that Gatsby is most likely one of the "everybody" who knows.
Fitzgerald gives us a second, more telling piece of evidence in Chapter 7 that Gatsby does indeed know of Tom's mistress. In this chapter, Nick and Gatsby go to visit Daisy in their residence on a particularly hot day. Just as they arrive, they hear the phone ring. They are then ushered into the salon where Daisy and Jordon are reclining on an "enormous couch." As Nick asks how they all are, they hear Tom's voice, "gruff, muffled, husky at the hall phone." Jordan has the audacity to whisper in Nick's and Gatsby's hearing, "The rumor is ... that that's Tom's girl on the telephone" (Ch. 7). Hence, even if Gatsby was not aware of the gossip at the start of the story, he is most certainly aware now.