General Nutrition Corp.--the nation's largest chain of health food stores--was fined $457,068 for destroying evidence in an antitrust case. The decision by federal Judge Cynthia Holcomb Hall came after the Pittsburgh-based corporation agreed that it would pay the entire fine itself. A former Superior Court judge, acting as a federally appointed special master, had recommended in 1982 that the Los Angeles law firm of Gibson, Dunn & Crutcher and a Pittsburgh law firm that also represented the corporation pay part of the sanctions because of the evidence destruction, which resulted in a default judgment against General Nutrition. In 1978, William T. Thompson Co., a Torrance vitamin and food supplement manufacturer, accused General of using bait-and-switch advertising tactics with Thompson products, a violation of federal law. General damages have not yet been determined. Gibson, Dunn & Crutcher denied any involvement.