Gatsby's father arrives to attend Gatsby's funeral. His character serves several functions. First of all, we learn what Gatsby was like as a boy. Mr. Gatz shows the notes that the young James Gatz wrote in his Hopalong Cassidy book, and we learn that Gatsby had big dreams very early and was driven to achieve: "He was bound to get ahead."
We also are reminded of Gatsby's modest beginnings. Mr. Gatz is a middle class man and was immensely proud of his son's financial accomplishments. In this way, more pathos is created for Gatsby who had dared to aspire to such heights only to be brought down by the sadly mistaken grieving husband, George Wilson.
Significance is a broad term. In my opinion, Mr. Gatz arrival symbolizes how alone Jay really was in his seemingly big life. Mr. Gatz knows very little, if any about the truth of Jay Gatsby's life today. The one person that came on the scene to honor the life of Jay was his father, an underinformed father. I think who Mr. Gatz was as a man demonstrates that life should be more about people and less about things. Obviously his connection with his son was of value to him, but for some reason it seems as if of late there has been great distance between the two. This should signal to readers that making the most of moments in life with those we care for is of utmost importance.