The Gannett newspaper company yesterday acquired the daily and Sunday newspapers in Wilmington, Del., for $60 million in cash - topping rival bids for the publications by the Hearst Corp. and The Washington Post Co., among others.

Gannett Co., based in Rochester, N.Y., immediately became owner of newspapers published by News-Journal Co., a former subsidiary of the giant E.I. du Pont de Nemours & Co., chemical complex.

Du Pont Co. invited bids for the publications last November, after Christiana Securities Co. was merged into the chemical firm. Christiana, previously an investment company owned by Du Pont family heirs, had owned News-Journal Co.

Newspapers involved in the transaction are the Morning News, with a circulation of about 50,000; the Evening Journal, 90,000 and combined News-Journals on Saturdays and Sundays. The newspapers together employ about 600 persons and take in annual revenues estimated at $25 million.

Du Pont Co. directors approved the transaction yesterday and the deal immediately was closed, giving Gannett 75 daily newspapers, the most of any American publisher.

In addition, Gannett's daily circulation reached 3 million with yesterday's sale, putting the Rochester company in a virtual tie with The Tribune Co. of Chicago, as the third-largest newspaper firm in terms of circulation.

The only companies with newspapers that have more daily circulation are Newhouse and Knight-Ridder. Currently pending is a Gannett agreement to buy the St. Thomas Daily News in the U.S. Virgin Islands, which would become No. 76 for the chain.

Du Pont Co. officials said the second highest bid for the newspapers was from Hearst, followed by The Post Co., the Mansfield (Ohio) Journal and Associated Newspapers Group, publisher of the London Daily Mail. Bids from the Gannett rivals were not made public, but a spokesman said Hearst and The Post Co. were with Gannett at "one end" of the price bidding with others "substantially below."

Irving Shapiro, chairman of Du Pont Co., said Gannett met fully his firm's requirements for a "responsible publisher with a proven record for quality journalism, sound employe relation policies, concern for the community and financial integrity."

Gannett officials, meantime, emphasized that the Wilmington papers will be run by autonomous local editors who will make editorial decisions. Gannett was unsuccessful in its attempt to retain Andrew Fisher as publisher of the papers, however. A new publisher will be selected later this week, according to a spokesman in Rochester.

Fisher said he wanted to follow interests he had before becoming publisher last summer. He will remain as a consultant during a transition period.

Gannett officials have said annual revenues for 1977 are expected to be about $550 million, up from $482.5 million last year. Annual profits have been forecast at $2.60 a share compared with $2.22 a year earlier.

Wilmington represents a new market for Gannett, which publishes papers across the country - including dailies in Camden and Bridgewater, N.J., Chambersburg, Pa., and a suburb of Pittsburgh.