To me, the old man is sort of a plot device -- he is used as a way to set up Gortsby to be fooled by the younger man. He also serves as the one who reveals to Gortsby that he has been fooled.
At the beginning of the story, the old man is setting next to Gortsby. When he leaves, the young man comes and the contrast between his dress and that of the old men help make Gortsby think the young man's story could be true.
After Gortsby has given the young man his money, the old man comes back and causes Gortsby to realize he has been tricked.
So the old man (unintentionally) sets Gortsby up to be fooled and then reveals (again not on purpose) that Gortsby has fallen victim to a scam.