The significance of Tom's gang to the story in Chapter 2 of Mark Twain's The Adventures of Huckleberry Finn is that its creation allows us to realize who is who in terms of social roles within Tom and Huck's inner circle. It also allows us to understand better the personality of Tom Sawyer, who still will mention the gang in The Adventures of Tom Sawyer. The name of the gang is, of course, "The Tom Sawyer's Gang", which lets us into the selfish and self-loving nature of Tom's behavior.
The gang, itself, does not last but for one month. They realize that since they had not robbed nor killed anyone, there is basically no real reason to maintain it going. Plus, there is Tommy Barnes among the so-called gangsters. This latter member is a very small child who cries easily after listening to the gang's oath. Therefore, even a mind as imaginative as Tom's would not be able to keep up a gang as macabre as the one he wants
Additionally, we see a gap in the relationship between Huck and Jim: Huck confirms to the readers that Tom loves to tell lies. This is significant because it shows that Huck's friendship to Tom surpasses the inconvenience of a bad habit, making their friendship seem even stronger.