How are the men in the novel shaped by the social classes in to which they were born? How does the social context and background of each of the male characters in the novel shape their behavior?

Social class plays an important role in shaping men in the novel. If we start with George Wilson, the most consistently lower-class male character in novel (I am leaving out the servants), he has been made abject and servile by poverty. He runs a gas station, is forced to live in the unpleasant Valley of the Ashes, and is dependent on the business of men like Tom Buchanan. Through much of the novel he is trying to wheedle business from Tom, who uses this as a means to see Myrtle, George's wife. Myrtle holds Wilson in contempt because he can't provide her with a comfortable lifestyle, and she feels justified in having an affair with Tom. We see how poverty "unmans" Wilson.
Tom, the most upper-class male in the novel, is a striking contrast to Wilson. He lives in a grand house by the water, owns a string of polo ponies, and strides through life as if he owns the world. He has inherited his great wealth, not worked for it, and he has a huge sense of entitlement. It means nothing to him to take Wilson's wife. Because he has always traveled in the most upper-class circles, he spots Gatsby as an impostor almost immediately and is immune to his attempts to impress people. Tom's insulated life allows him to be brutal, racist, and classist. He takes for granted that he has the right to feel superior to the rest of the world. If Wilson's lower-class status means he creeps through life like a shadow, Tom dominates the space he occupies, wrecking lives as he goes.
Like Wilson, Gatsby comes from poor roots. While we are told very little about Gatsby's background, Tom spots him immediately as a not a "Nordic" and hence inferior. Gatsby's sense of shame over his poverty drives his ambition and makes him willing to embrace a life of crime to get ahead. Part of his desire for Daisy stems from her wealth; she has never known any want and is the living embodiment of the grace, beauty, and charm that a privileged life offers. When she falls in love with Gatsby, it signals to him that he can rise above his poverty. But he never can quite do it. For all his vast wealth, there are gaps that reveal his past: he wears a pink suit, which is a bit tacky; talks about big game hunting in Europe, which has no "big game" of the sort people went after in Africa and India; and the books in his library have uncut pages, meaning he hasn't read them. His lavish house and parties can't obscure that he comes from humble roots and doesn't quite belong in the upper-class world he pretends to be a part of. In the end, Daisy opts for the safety of the real thing: Tom.
Nick, like Tom, comes from comfortable roots. He has nowhere near Tom's vast wealth, but his family is able to send him to an elite college and bankroll him as he begins in the financial sector in New York City. He can afford to rent a house with a servant. His lifestyle seems quite modest, but that is only in contrast to people like Tom and Gatsby. Because of his privilege, he has been taught by his father not to judge other people—he has been bred in the courtesy of the upper-classes. Most tellingly, he is immediately accepted by Tom as one of his set: not only is Nick a college friend of Tom's and Daisy's cousin, Tom refers to him as a Nordic. He never treats Nick with contempt. Nick's place as an upper-class insider on the periphery, with not much money compared to the superrich, gives him more compassion and sympathy for a climber like Gatsby than a man such as Tom Buchanan can muster.