What is the significance of mr. gatz’s arrival?

We know very little about Gatsby's true family because he claims that they all died before he moved east. In the last chapter, it is revealed that Gatsby has 1 remaining family member, which is his father. The readers learn more about Gatsby's true past through his father and receive a new perspective of Gatsby from a father's point of view. 

Proud:  
"He had a big future before him, you know. He was only a young man but he had a lot of brain power here" (Fitzgerald 168). This was said by Mr. Gatz and illustrates that he believed that his son would be successful. Mr. Gatz takes pride in what his son has been able to achieve and how he has been able to rise into the wealthy high class. 

Understanding:"He come out to see me two years ago and bought me the house I live in now. Of course we was broke up when he run off from home, but I see now there was a reason for it. He knew he had a big future in front of him. And ever since he made a success he was very generous with me" ( Fitzgerald 172). Despite Gatsby running away from home, and leaving his parents, Gatsby's father is understanding of his choices to pursue a better lifestyle. 

Materialistic: 
"I went next door and found Mr. Gatz walking up and down excitedly in the hall. His pride in his son and in his son's possessions was continually increasing" (Fitzgerald 172). Mr. Gatz is a very proud father because he has based his pride on what his son has been able to achieve in terms of wealth, however, little does he know Gatsby was never really successful and happy because he failed to secure his future with Daisy. 

Cold: 
"Well, the fact is -the truth of the matter is that I'm staying with some people up here in Greenwich, and they rather expect me to be with them tomorrow. In fact, there's a sort of picnic or something." (Fitzgerald 168). Mr. Gatz travels all the way to New york for his son's funeral, although, attending Gatsby's funeral is not his primary concern, rather a picnic with some of his friends is more important. It shows an insensitive side to Mr.Gatz.

Oblivious: "If he'd of lived, he'd of been a great man. A man like James J. Hill. He'd of helped build up the country" (Fitzgerald 168). This shows that Mr. Gatz is  oblivious to how his son got his money through illegal business and how he became a part of the high class. He is  also oblivious as to how unhappy his son was.