Gildo was a Berber by birth. Being a son of military officer Nubel, he was brother to Firmus. When Firmus revolted against Valentinian I (375), Gildo stayed loyal to his emperor and, at the suppression of the revolt, was awarded with the immense patrimony confiscated from his brother. His name means ruler in the Lybico-Berber language and is connected to the modern-Berber word for king, Agellid.
In 386, Theodosius I appointed Gildo Comes Africae and Magister utriusque militiae per Africam, as reward for his support to his father Theodosius the Elder in the suppression of Firmus' revolt. The Africa Province was ruled by Gildo with some sort of independence, and oppressed by every species of tyranny.
After the death of Theodosius and the rise to the throne of his sons, Arcadius and Honorius, Gildo saw an increase in his importance: the Africa Province, in fact, became entrusted with the grain supply to the city of Rome, a role played by Egypt until the split of the Empire into two halves. Incited by the political machinations of the eunuch Eutropius, Gildo seriously entertained the notion of joining the Eastern Roman Empire by pledging fidelity to Arcadius. The possibility of losing the granary of Rome led to civil turmoil in the city, and acting on an appeal by Stilicho, the Roman Senate declared Gildo an "enemy of the State" and started a war against him.
At the same time, Gildo had a quarrel with his brother Mascezel, who was obliged to flee to the court of Honorius; Gildo took revenge on his brother by killing his two sons. Mascezel was entrusted by Stilicho with the command of the war against Gildo and given the command of a chosen body of Gallic veterans, who had lately served under the standard of Eugenius. Landed with his few troops in Africa, he camped in front of a large Moorish camp. Despite its superiority, Gildo's army melted in front of the superior discipline of Mascezel's army.
After the defeat, Gildo was obliged to flee in a small boat, hoping to reach the friendly shores of the Eastern Roman Empire. However, unfavourable winds drove the vessel back to the harbour of Tabraca, where the inhabitants were eager to display their new loyalty by throwing Gildo in the dungeons. To avoid the revenge of his brother, Gildo committed suicide by hanging.
- Gibbon reports the units that formed the 5,000 soldiers strong military body:
These troops, who were exhorted to convince the world that they could subvert, as well as defend, the throne of an usurper, consisted of the Jovian, the Herculian, and the Augustan legions; of the Nervian auxiliaries; of the soldiers who displayed in their banners the symbol of a lion; and of the troops which were distinguished by the auspicious names of Fortunate and Invincible.
— Edward Gibbon, The Decline and Fall of the Roman Empire
- Platts, John, s.v. "Gildo", A New Universal Biography, Sherwood, Jones, and Co., 1826