Instructed by the Spirit, Alma cries repentance to the Gideonites. He had certainly urged repentance on the Zarahemlaites, but they clearly needed repentance more drastically than the people of Gideon. In what sense then, must the Gideonites repent?
Repentance does not require great sins. It does not even require sins of commission. A call to repentance can come to any who are able to improve. Thus, the Gideonites were good people yet still capable of improvement. In particular, Alma is asking them to prepare for the Messiah’s coming—a real coming in the not-far-distant future. These people are literally to “prepare the way of the Lord.”
Reference: Probably Alma assumes that his listeners would recognize the echoes of Isaiah: “The voice of him that crieth in the wilderness, Prepare ye the way of the Lord, make straight in the desert a highway for our God” (Isa. 40:3). Mark 1:3 identifies John the Baptist as “the voice of one crying in the wilderness, Prepare ye the way of the Lord, make his paths straight” (see also Luke 3:4). Like other passages, New Testament language probably influenced Joseph Smith’s translation here. (Confirmation that Alma refers to Isaiah comes in Alma 7:19.)