Rosenstock’s father George was a doctor and writer from Schleswig-Holstein, Germany, who served as a medical officer with the Wehrmacht in World War II. His mother was a nurse from County Galway. Gabriel was the third of six children and the first born in Ireland. He was educated locally in Kilfinane, then in Mount Sackville, County Dublin.
He exhibited an early interest in anarchism and was expelled from Gormanston College (County Meath) and exiled to Rockwell College (County Tipperary). Later, he attended University College Cork.
His son, Tristan, is a member of the Irish traditional music quintet Téada, and impressionist/actor Mario Rosenstock is his nephew.