__I '(Church Organist Murder Draws'20-Year Sentence.FORT WORTH (AP) - Jurors have decided Jerry . Don Goss, 24, must, serve 20 years in prison, for the killing of a church organist last fall.He was convicted Thursday in the slaying of Elza Cook, 63, in Cook:s apartment here Sept.5, 1973.Starting Wednesday, a jury took three hours to agree on a verdict of murder with malice i I and two hours longer to reach a decision on the penalty.Goss claimed he killed Cook in self defense. A minister found the body of Cook in the apartment after the organist failed to appear for a service at St. Stephen Presbyterian . Church.Testifying Wednesday, Goss said ho met Cook at an X-rated movie house in March last year i and went with him to engage in rex at the apartment. He said i they met again at the theater J the evening of the slaying, Cook threatened him with a knife and they had another sex parly-