Gordon Gahan, 38, a former area resident who had been a staff photographer with both United Press International and the National Geographic magazine before starting his own advertising photography business in New York in 1982, was one of three persons killed Oct. 19 in a helicopter crash in the Virgin Islands.

The helicopter's pilot, Steve Booker of St. Thomas, and Mr. Gahan's assistant, Joseph Capitelli of New York, were identified by wire services as the other two persons killed. News services said the three were taking aerial photographs of sailboats at the time.

A yacht captain, Jim Snodgrass, was the only witness to the crash, and told the Virgin Islands Daily News that the helicopter crashed and sank in about 65 feet of water, about 100 feet from his vessel.

Mr. Gahan, who was a former Bethesda resident, was a native of New York City where he had attended Columbia University. He served with the Army, spending 1967 in Vietnam as a photographer for the Army Digest. His medals included two Bronze Stars and a Purple Heart.

After the war, he joined UPI, serving as its Minneapolis bureau manager, before becoming a photographer with the National Geographic here in 1972. Over the next 10 years, assignments took him to the South Pacific, Africa, South America, the Mediterranean, and behind the Iron Curtain. Two years ago, he left the Geographic to become coowner of Prism Photography Inc. in New York.

Mr. Gahan was a member of the White House News Photographers Association and the National Press Photographers Association.

Survivors include his mother, Alice Gahan of Fort Lauderdale, Fla., and a sister, Barbara Robinson of Greenwich, Conn.