One shaft of sunlight that has illuminated a rather gloomy year for most Americans has been the year-long celebration in honor of George Gershwin. When Gershwin died suddenly at age 38 in 1937, the novelist John O'Hara wrote in his typically hard-boiled-sentimental style: "They tell me George Gershwin is dead, but I don't have to believe it if I don't want to." Corny or not, O'Hara's sentiments seemed to have been shared by many this year, as the many television specials, new recordings and stagings of Gershwin's work attest. In fact, the 50th anniversary appears to be closing with a rush this December as one Gershwin gala crowds another throughout the country.