The sub-prefectures of the department are Bayota, Dahiépa-Kéhi, Dignago, Dougroupalégnaoa, Doukouyo, Gagnoa, Galebre-Galébouo, Gnagbodougnoa, Guibéroua, Ouragahio, Sérihio, and Yopohué.
Using current boundaries as a reference, from 1969 to 1980 the department occupied the same territory as Gôh Region.
In October 2005, Gagnoa Department was the base of Bangladeshi UNOCI troops aiming to secure the proposed presidential elections in the country.
At this time, Gagnoa Department became part of Gôh Region in Gôh-Djiboua District.
The population of the department is predominantly from the Bété and Gagu ethnic groups.