Born in Breslau, Gehlhaar was the son of a German rocket scientist, who emigrated to the United States in 1953 to work at a rocket-development research centre in New Mexico.
[2] He moved to Cologne, Germany, in 1967 to become assistant to Karlheinz Stockhausen, and became a member of his performing ensemble.
[1] In 1969, together with Johannes Fritsch and David C. Johnson, he founded the Feedback Studio, Cologne, a new-music performance center and publishing house.
He later moved to England, where he became in 1979 a founding member of the Electro-Acoustic Music Association of Great Britain.
His works are for both acoustic and electro-acoustic media, though he is best known for his work with computer-controlled composition, and for his interactive installations such as Sound=Space (1985), HeadSpace (2000), CaDaReMi (2006), Walking on Earth (2007), and has for many years collaborated with Luis Miguel Girao of Artshare, Aveiro, Portugal.