In chapter six of The Great Gatsby, by F. Scott Fitzgerald, Nick Carraway reveals to the readers what he knows about Gatsby's past. The young Jimmy Gatz of North Dakota left his miserably poor home to begin a new life at the age of sixteen; a year later he had already decided who he would become when the opportunity presented itself for him to become a new person. He eventually meets Dan Cody and becomes Jay Gatsby.
The quote you mention in your question describes Jimmy Gatz in the year between his leaving and his re-creation. He spent that year working
along the south shore of Lake Superior as a clam-digger and a salmon-fisher or in any other capacity that brought him food and bed. His brown, hardening body lived naturally through the half-fierce, half-lazy work of the bracing days. He knew women early, and since they spoiled him he became contemptuous of them, of young virgins because they were ignorant, of the others because they were hysterical about things which in his overwhelming self-absorbtion he took for granted.
This description gives the sense of a young boy kind of drifting along and trying to get by as he waits for something that will change his life. He is concerned about making his own way and therefore concerned only about his own pleasures. This is the sense of his "overwhelming self-absorption"--taking whatever pleasures he can because that is all that matters to him. Jimmy Gatz cared nothing for anything or anyone but himself; that is the definition of overwhelming in this passage.
Self-absorption is actually not a trait Gatsby loses when he is older. Everything he does is designed to bring him pleasure. Though his primary goal is attracting Daisy, even that is for his own satisfaction and pleasure. There is a certain narcissism in his pursuits and in the persona he has created, something he obviously learned early and practiced often throughout his life.