Gabriel was born in Ireland in 1950, the eldest of six children. His father was a cooper and his mother a hospital worker. He attended University College Dublin, studying archeology and linguistics, and followed this with work as an archaeologist, cook, bullfighter and Spanish school teacher. He got into acting at 29, when he joined the London Royal Court Theatre.

Gabriel has two children (Jack Daniel, b. 1989 and Romy Marion, b. 1992) with Ellen Barkin, the actress, and his ex-wife, whom he met on the set of Siesta in 1987. They divorced in 1999, following an amicable separation.