Charles received his medical educated at the School of Physics, Trinity College, Dublin, where he took the degree of M.D. in 1907. As a student he played for the University Hockey Team, and he was the first medical officer to be attached to the medical unit of the Dublin University O.T.C. - it was said that it was due to his ‘energy, popularity, and ability’ that the unit became a real success in its early days, - sending many men into the R.A.M.C. Charles joined the R.A.M.C. himself on 1st August 1908 as a lieutenant, and was promoted to captain on 1st February 1912. When war broke out, he was serving in India, where he remained until he was sent to Mesopotamia, arriving on 1st May 1917.
He was the eldest son of Henry Gregg (F.R.C.S.I., Professor of Dentistry at the Royal College of Surgeons, Ireland) and Katie Sherlock of Glenageary, Co. Dublin; and the husband of Margaret Murray Ross Sherlock of Summerfield, Dalkey, Co. Dublin.