Geography: For the city of Gideon to be in the valley of Gideon rather requires that the valley of Gideon be a different valley from the greater valley in which we find the river Sidon. Sorenson suggests that this is a valley on the Eastern side of the mountain ridge ringing the valley of Zarahemla (see Sorenson, 1985 p. 154, Map 7. See Sorenson, John L. The Geography of Book of Mormon Events. FARMS1990, p. 246 for information on distances from the valley of Gideon to Zarahemla).
It is perhaps tempting to think that the valley and city of Gideon were named for Gideon after his death at the hand of Nehor, since we do not hear of these names until after Gideon's death. Since the names appear in the fifth year of the reign of the judges, and Gideon is slain in the first year of the reign of the judges (Alma 1:1), it is much more likely that the valley of Gideon was given to him and his lineage when they arrived from the land of Lehi-Nephi.
It is unlikely that the valley and city would have been begun and become well established in only five years. Indeed, it is possible that Gideon was slain in the valley of Gideon. That valley would surely have been his home, and Nehor may have been attempting to gain influence in the outlying areas before working his way in to Zarahemla proper.