Gannett Co. Inc. yesterday reported net income of $89.7 million (56 cents a share) for the second quarter of 1987, up 16 percent from $77.2 million (48 cents) in the same period the previous year. Operating revenue was $789.8 million in the second quarter, compared with $716.2 million for the same quarter in 1986. Operating income grew to $181.7 million from $168.1 million. Nonoperating income included a gain on the sale of a Detroit radio station, WCZY-AM and FM. Earnings before taxes totaled $166.1 million, compared with $150.3 million in 1986.

For the first half of 1987, Gannett's net income increased 17 percent to $142.3 million (88 cents) from $122 million (76 cents) earned in 1986. Operating revenue totaled $1.5 billion in the first half of 1987, an increase of 12 percent over $1.3 billion in 1986. Per share results for 1986 were restated to reflect a 2-for-1 split in January 1987.

John J. Curley, president and chief executive officer, said that the newspapers reported sharply higher profits in the second quarter. Newspaper lineage was up 4 percent, reflecting strength in classified advertising. At USA Today, advertising pages numbered 1,116 in the second quarter, up 20 percent over a year ago.

Broadcasting revenue increased slightly in the second quarter, including the results of radio stations purchased in 1986. Outdoor revenue was lower in the quarter. Gannett owns 90 daily newspapers, 39 nondaily newspapers, 8 television stations, 16 radio stations and an outdoor advertising company.

Signet Banking Corp. reported a loss of $40.2 million ($1.60 a share) for the second quarter because of its decision to increase its loan loss reserve by $75 million to cover potential losses on its loans to less developed countries.

The increase brings the second-quarter total reserve allowance to $158 million, or 2.4 percent of total loans. The loss for the quarter, ending June 30, compared with net income of $21.2 million (87 cents) for the same period the previous year.

The loss for the first six months of the year was $18 million (72 cents), down from a net income of $40.1 million ($1.67) for the first half of 1986.

Total assets on June 30 were $10.5 billion, up 23 percent from the $8.5 billion a year before. Deposits increased 27 percent to $7.2 billion from $5.7 billion. Signet is a multistate bank holding company comprised of Signet Bank/Virginia, Signet Bank/Maryland and Signet Bank N.A. in the District of Columbia.

Citizens Savings Bank F.S.B., of Silver Spring reported earnings for the second quarter of $375,000 (18 cents), down 43 percent from $666,000 the same period in 1986. Earnings were reduced by a $382,000 (19 cents) after-tax expense because of the Federal Home Loan Bank Board's elimination of its insurance fund's secondary reserve in May.

Earrnings for the first half of 1987 were $1.67 million (85 cents), a 13 percent increase from $1.48 million in the first half of 1986. Earnings for the six months included a $511,000 (26 cents) after-tax gain following termination of the bank's former pension plan.

On June 30, the bank's assets totaled $452.4 million, a 7 percent increase over the Dec. 31 level.