Gerhardt was the son of an Alsatian chemical manufacturer from Strasbourg. He was educated at the universities of Karlsruhe, Leipzig, and Giessen where he studied under Justus von Liebig. From 1838 he worked in Paris as assistant to Jean Dumas before becoming professor of chemistry at Montpellier (1844). He returned to Paris in 1848 and worked with Auguste Laurent in their private laboratory until he was appointed to the chair at Strasbourg in 1855. He published two original works: Précis de chimie organique (1844–45; Summary of Organic Chemistry) and his Introduction à l'étude de la chimie par le système unitaire (1848; Introduction to the Study of Chemistry by Means of the Unified System).

Gerhardt is best known for his attempts to rationalize organic chemistry. Like most chemists he was aware that the dualistic system of Jöns Berzelius was unsatisfactory and tried to create an alternative. He adopted what became known as ‘type theory’ in which he saw all organic compounds with reference to four ‘types’ – hydrogen, hydrogen chloride, ammonia, and water. Organic compounds were referred to these types by replacing a hydrogen atom in one of these compounds by a radical (i.e., by a group of atoms).