One of Gatsby's dominant characteristics is the contradiction between his gullible innocence and his calloused experience. While he is a man who has earned a fortune from illegal activities with formidable underworld characters like Wolfshiem, Gatsby nonetheless innocently holds onto his gullible dream that the green light at the end of the dock holds his lost love out to him like a beacon of hope. The experienced man of the world (indeed, the underworld) conflicts with the man who believes that lost love will be reclaimed by simply approaching the subject of his affection, Daisy, in a similar way to which he might approach the green light and claim it, a physical object.
[I saw] the colossal vitality of his illusion. It had gone ... beyond everything. He had thrown himself into it with a creative passion, ... decking it out with every bright feather that drifted his way.
Another dominating characteristic is his capacity for vigorous dedicated work and for accepting opportunities that come his way. Not everyone who is offered an opportunity can see it as an opportunity, while others may see yet be unable to accept--to act upon--the opportunity. That Gatsby could see and accept opportunity and immerse himself in whatever conditions called for from him is a characteristic that helps propel him to his elevated position, a position he hopes will bring Daisy back to his arms (perhaps single-mindedness is another dominating characteristic).
For over a year he had been beating his way 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.