Gideon appears abruptly in our story. We are not given any of his background, but from his actions as they unfold we can easily surmise that Gideon is a military man. Verse 22 below will speak of the "men of Gideon," clearly placing Gideon as a recognized leader of men. Mosiah 20:21 clearly shows that Gideon has a knowledge of the prophecy of Abinadi concerning Noah and his people.
These small pieces of information can give us a probable picture of Gideon prior to this sudden introduction. As a man of action and the sword, we can place Gideon in the military. His actions are not those of a man given to political intrigue, but of physical action. He must have been part of the military assigned to the palace, as his knowledge of Abinadi's statements would indicate that he either heard them directly, or through the world of mouth information exchange that would have been part of palace life.
His dissatisfaction with Noah, combined with the citation of Abinadi, suggests that he was one of the men who left Noah's military service and would have been aligned with those who would have been the follower's of Zeniff suggested above. We may further surmise that he was a captain of men (or we may call him a general - we don't have any information on rankings in Noah's military - it is sufficient that he was a commander). It is entirely likely that he took with him all of his military unit n his defection, accounting for the designation "men of Gideon" as well as a fair part of the reduction of Noah's military.