He was head of the BND, Germany's Federal Intelligence Service from 1 May 1968 to 31 December 1978.
According to his obituary in the Los Angeles Times, entitled Gerhard Wessel, 88; Did Espionage Work for Hitler, West Germany, the "former intelligence officer in Adolf Hitler’s anti-Soviet spy operations" ... "is credited with modernizing the BND by hiring academic analysts and electronics specialists".
[1] The obituary issued by the New York Times News Service summarized some of the BND's successes under Wessel: "It informed the government three months in advance of the Soviet Union's plans to invade Czechoslovakia in 1968.
It also gathered early information about dissatisfaction among shipyard workers in Gdansk, which eventually led to upheaval in Poland in the 1980s".
[2] Wessel was born in the Holstein city of Neumuenster; his father was an Evangelical pastor.