Gallant was born in Montreal, Quebec, the only child of Albert Stewart Roy de Trafford Young, a Canadian furniture salesman and painter who was the son of an officer in the British Army, and his wife, Benedictine Wiseman. Young died in 1932 of kidney disease, and his widow soon remarried and moved to New York, leaving their daughter behind with a guardian. Gallant did not learn of her father's death for several years and later told "The New York Times": "I had a mother who should not have had children, and it's as simple as that."