Gatsby's father tells Nick about Gatsby and what he was like as a young man. The ambition and the innocent striving for "betterment" stand as an interesting and telling background for a character who refuses to associate with his past.
Gatsby's father might talk about the innocence that Jay Gatsby has disassociated himself from and speak about the person who was simple-hearted enough to make a list of his daily tasks for personal improvement.
Certainly, the reader already knows that Henry Gatz thinks the world of his son. He brings out the title that he would have been a great one. Yet, if the assignment is to deliver a possible eulogy, I think that it would bring a great deal of emotion to the forefront if Gatz laments the loss of his son as part of an indictment of the time period and the social setting in which Gatsby wanted acceptance. The copy of Hopalong Cassidy that the father brings could be a great starting point for this speech. I think that Gatz, as a grieving father, could lament the loss of his son, but I also think that a speech could be written whereby he understands the callous nature of a Tom or Daisy, and the dangerous frivolity of the flapper scene. Gatsby is a victim of this setting, the one where people are used as a means to an end and not as ends in their own right. I think that Gatz would have had this picture drawn to him as a result of his conversations with Nick, and having the father deliver a stinging rebuke of all that this society embraces with its ultimate price paid by Jay Gatz would be a wonderfully poignant speech of a father lamenting the loss of his son for such a silly end.