Cecily Truett and Larry Lancit rolled the dice. In the spring of 1991, they took their production company and its best known product, and laid them at the feet of GKN Securities Corp., a small investment firm, which organized the initial public offering of their production company. By then, the Lancits had filled a trophy case with awards as producers of Reading Rainbow. But Lancit Media Productions’ earnings were barely enough to scrape by. It was certainly not enough to expand.