Gilpatric was born in 1906 in Brooklyn, the son of Wall Street attorney Walter Hodges Gilpatric, an Amherst College graduate born in Warren, Rhode Island, and the former Charlotte Elizabeth Leavitt, a graduate of Mount Holyoke College, born to American missionary parents in Osaka, Japan. Charlotte Leavitt was a college classmate and lifelong friend of Frances Perkins, the first woman appointed to a Presidential Cabinet. On his mother's side Roswell Gilpatric was related to Harvard College astronomer Henrietta Swan Leavitt, whose father was the Congregational minister George Roswell Leavitt.