Answer and Explanation:
It is Jay Gatsby himself who calls Nick "Old Sport" in the novel "The Great Gatsby", by F. Scott Fitzgerald. Gatsby had remained as this mysterious figure throughout the story until he finally introduces himself to Nick during a party at Gatsby's mansion.
The term "old sport" was a way gentlemen had to address one another at the beginning of the 20th century. By using it, Gatsby has readers infer that he truly wants to be friends with Nick. He treats Nick as his equal, as his friend, even though they had never met before. We can infer Gatsby has an affectionate, easy-going manner.