Verse 6 confirms that Gidgiddoni had established a scorched earth policy which was aided by occasional sorties that could disrupt any attempts to grow food. Thus, if the Gadiantons were to accomplish their desires, they had to conquer the Nephites.
They attack in the nineteenth year, the sixth month. Gidgiddoni’s withdrawal into the north began near the end of the seventeenth year (3 Nephi 3:22). That withdrawal would have taken some time. Thus, this attack comes somewhere around a year after that withdrawal had been accomplished.
When the Gadiantons attack, “they had a lamb-skin about their loins, and they were dyed in blood, and their heads were shorn.” We cannot know precisely what all of this looked like. For one thing, there were no sheep, so we aren’t sure what the reference to the lamb-skin might have been. We can be comfortably certain that a shorn head meant a shorn head, but the reason might be cultural. In Mesoamerica, captives are often shown being grasped by a shock of their hair. Perhaps the shorn head would signal that this would be a fight in which there would be no captives. We can also expect that whatever was died in blood (the text could refer to the lamb-skins dyed in blood, or even the warriors), that was a visible symbol that was intended to give fright to the enemy.