I think it was on the the third day that a telegram signed Henry C. Gatz arrived from a town in Minnesota. It said only that the sender was leaving immediately and to postpone the funeral until he came.
This is the first that the reader learns of Jay Gatsby's parents. There is no mention of Mrs. Gatz, and when Gatsby's father arrives he is bundled in a great coat against what Nick describes as a "warm September day." His "eyes leaked continuously with excitement" as Nick escorts him to the music room and seats him. After he has calmed down, he tells Nick that he read of his son's death in the Chicago papers. Then Nick shows him to the room where his son is; when Mr. Gatz steps out, he finally looks around and his grief changes to "an awed pride." Then, after telling Nick that Jay visited him two years ago, he pulls out a worn photograph of the house that Jay sent him; obviously, he has displayed this photograph many times. He proudly shows Nick a book in which the young Gatsby wrote his daily schedule, and Mr. Gatz reads it to Nick.
By the time the minister arrives, there are no other people. "Nobody came." Only Nick and Mr. Gatz and a couple of servants drive to the cemetery where Owl Eyes follows. It is only Mr. Gatz who has been impressed with Jay Gatsby's tremendous ambition, and he is the only family member there, the only one who loved Jay; it is only Owl Eyes who respects Gatsby as he remembers his genuine gestures, such as filling his library with real leather bound books.
He represents Gatsby's reality. He is not fancy. He is simple. Although Gatsby presents himself as larger than life (and almost mythical), his father's arrival is something that reminds us that he is just as "average" as the rest of us.
He also represents Gatsby's past (which Gatsby tried to forget/hide) - and shows us that we will always be connected to our beginnings in some way.
The entire book is filled with symbols and symbolism. Gatsby's father is one of the symbols, much like Daisy, who are characters in the story but also represent something greater. While on the surface Mr. Gatz seems to be just a long lost father mourning the death of his son, he represents, much like what answer #2 said, everything Gatsby tried to hide, push away, or forget about. He represents the past Gatsby tried to forget, as well as the fact that he can never truly forget his past.