How is the comparison of Gatsby with Christ ("he was a son of god... and he must be about his father's business") ironic?
Comparing Gatsby with Christ is ironic for several reasons. Here are three.First, Christ's father always knew what Jesus was doing. Gatsby's father does not know his business. Second, Jesus was selfless. Gatsby lacks a self: he is recreating himself, trying to fill a tremendous and painful gap or hole. Third, Jesus brought a mission and a message to others; he had followers. Gatsby had people who took advantage of him, but his influence evaporated once he was dead. Jesus was more influential after death; Gatsby's influence dies with him.
The quotation that calls Gatsby a "son of God" is ironic on several levels.
To begin with, although Gatsby is "a son of God" and removed from the merely human realm in the sense that he has dedicated himself to something outside himself, an ideal, the ideal is not very exalted. As the quotation goes on to say, he has put himself in the service, not of humanity as a whole, but "a vast, vulgar, and meretricious beauty."
The second and more profound level of irony is in the way the quotation foreshadows Gatsby's end. He will soon be killed by George, Myrtle's husband, who has been told by Tom that Gatsby was driving the car that killed Myrtle. In other words, Gatsby will die in the service of his ideal as a result of an unjust judgment, ironically evoking the sacrifice of Jesus on the cross. A martyr to his mundane ideals, Gatsby ends up assuming the responsibility and paying the price for the sin of someone else, Daisy.