 In those years, according to a case study by the Harvard Business School, H&K's clients produced nearly 10% of the GNP.Gray provided services to accounts that included the American Petroleum Institute, Procter and Gamble, and the National Association of Broadcasters and El Paso Natural Gas.El Paso hired Hill and Knowlton to drum up support for legislation that would allow El Paso to buy out its competitor, Pacific Northwest Pipeline Company.