After toppling the Nephite government and establishing their own king over a much smaller people, Jacob, the new king, decides that they have to go north. When they were strong enough to survive harassment on the journey, they leave the land and continue north, “out of the reach of the people.” For all we can tell, those tribes who had previously been Nephite, remained in the land of Zarahemla.
Even though Mormon doesn’t tell much of this Gadianton king, or what they did, he makes certain to explain that the Gadiantons head north. Conceptually, he is tying the Gadiantons (those who destroyed the Nephites just before the Messiah appeared) to the north, where the Gadiantons, who destroyed the Nephites of Mormon’s days, came from.