Rosenstock's father was a doctor and writer from Schleswig-Holstein; his mother a nurse from County Galway. Gabriel was the third of six children and the first born in Ireland. He attended Rockwell College and the National University of Ireland, Cork.
Rosenstock worked for some time on the television series Anois is Arís on RTÉ, then on the weekly newspaper Anois. Until his retirement he worked with An Gúm, the publications branch of the North-South body which promotes the Irish language, Foras na Gaeilge.
Although he has worked in prose, drama and translation, Rosenstock is primarily known as a poet. He has written or translated over one hundred books.
Rosenstock is a member of Aosdána, and a former chairman of Poetry Ireland. Two of his more recent works are Eachtraí Krishnamurphy (2003) and Krishnamurphy Ambaist (2004). His travel book Ólann mo mhiúil as an nGainséis (2003) provides an insight his beliefs, intellectual life, and philosophy.
His son Tristan Rosenstock is a member of the traditional Irish quintet Téada while his nephew is Mario Rosenstock.