GTE Corp. has become the nation’s largest utility.

GTE had been a perennial second to giant American Telephone & Telegraph but stepped up to the No. 1 position as a result of the breakup of AT&T;, Fortune magazine said in announcing its rankings of the 500 largest service companies in America. When AT&T; shed its regional telephone companies last year, it moved out of the service category, and earlier this year it made its debut on the Fortune 500 list as the eighth-largest industrial concern in the nation on the basis of sales.