Harmon Gow is the town gossip. He used to drive the stagecoach before the railroad was built and that is how he knows the stories of all the families in town. Harmon is the first person to give the narrator information about Ethan, and the person who suggests that the narrator hire Ethan. If the narrator hadn't hired Ethan he would never have gotten to spend the night at Ethan's house. This means, of course, that he would never have learned the secret that allowed him to imagine the story.
So, even though Harmon Gow drops out of the picture after the Prologue, he is an important character. He connects Ethan and the narrator, and he is our reminder to never forget that this is a story. Unless Ethan or Mattie told every detail of their relationship that night, the narrator couldn't have heard about Mattie and Ethan's private moments. Those moments are in "the gaps" (Prologue.16) and can only be imagined.