Gatsby's father is mourning the way in which his son's life had played itself out, expressing regret for all the promise that Jimmy (Jay's name as a boy) had shown but was now not going to be able to realize. He refers to James J. Hill as an example of a "great man," someone who had "helped build up the country" and comments that his son could have become someone in a similar position if he had lived.
James J. Hill made a huge fortune for himself as chief executive officer and primary stockholder of railroad lines that built transcontinental train travel into a huge business. Hill built a near monopoly in control of companies serving different rail routes in partnership with J P Morgan, taking advantage of every possible means to avoid government control or taxation of his profits. His actions led to great personal wealth, but his building of the country was not the philanthropic activity Mr. Gatz assumed.