Gallus was born in Italy, in a family with respected ancestry and senatorial background. He had two children in his marriage with Afinia Gemina Baebiana: future emperor Gaius Vibius Volusianus and a daughter, Vibia Galla. His early career was typical with several appointments, both political and military. He was suffect consul and in 250 was nominated governor of the Roman province of Moesia Superior, an appointment that showed the confidence of emperor Trajan Decius in him. In Moesia, Gallus was a key figure in repelling the frequent invasion attempts by the Gothic tribes of the Danube and became popular with the army.
On July 1 251, Decius and his co-emperor and son Herennius Etruscus died in the battle of Abrittus, at the hands of the Goths they were supposed to punish for the raids. When the army heard the news, the soldiers proclaimed Gallus emperor, despite Hostilian, Decius' surviving son, ascending the imperial throne in Rome. Gallus did not back down from his intention to became emperor, but accepted Hostilian as co-emperor, perhaps to avoid the damage of another civil war. While Gallus marched on Rome, an outbreak of plague struck the city and killed the young Hostilian. With absolute power now on his hands, Gallus nominated his son Volusianus co-emperor.
Eager to show himself competent and gain popularity with the citizens, Gallus swiftly dealed with the epidemics, providing burial for the victims. Gallus is often accused of persecuting the Christians, but the only solid evidence of this allegation is the prison of pope Cornelius in 252.
Like his predecessors, Gallus did not have an easy reign. In the East, king Shapur I of Persia invaded and conquered the province of Syria, without any response from Rome. In the Danube, the Gothic tribes were once again on the loose, despite the peace treaty signed in 251. The army was not pleased with the emperor and when Aemilianus, governor of Moesia Superior and Pannonia, took the initiative of battle and defeated the Goths, the soldiers proclaimed him emperor. With an usurper threatening the throne, Gallus prepared for the fight. He recalled several legions and ordered reinforcements to return to Rome from the Rhine frontier. Despite the dispositions, Aemilianus marched onto Italy ready to fight for his claim. Gallus did not have the chance to face him in battle: he was murdered by his own troops in August 253. His son Volusianus shared his fate.