Charles Gustave Agate was born in Pendleton, Lancashire, England on October 1878 and was articled to William Alfred Royle (1839?-1904) and Robert Isaac Bennett (c.1841-1901) of Royale & Bennett from 1897 to 1902. He remained as their assistant until 1903. During these years he attended Manchester School of Art and Techical School. In 1903-04 he worked as an assistant to Richard Barry Parker (1867-1947) and Raymond Unwin (1863-1940) of Parker & Unwin.
Agaate commenced independent practice as an architect in 1905 and in 1916 he became a partner in the architectural practice Halliday & Paterson, formed in 1912 by James Theodore Halliday (1882-1932) and Claude Paterson (1883-1950) which thus became Halliday, Paterson & Agate. In 1926 Paterson withdrew from the partnership which then became Halliday & Agate. James Theodore Halliday's younger brother Franklyn Leslie Halliday also became a partner in the practice in 1926. The partnership was dissolved following the death of James Theodore Halliday in 1932.
Agate was elected a Licentiate of the Royal Institute of British Architects (LRIBA) in 1911. He died in Middleton, Lancashire on 25 June 1959
Directory of British Architects 1834-1914. Compiled by Antonia Brodie, et al. Volume 1: A-K. London; New York: British Architectural Library, Royal Institute of British Architects/Continuum, 2001