He played a major part in the Second Punic War, establishing Roman rule in the east of the Iberian peninsula and tying up several Carthaginian armies to keep them from reinforcing Hannibal.
In 218, Gnaeus and his brother sailed with Publius' consular army to the allied Greek city of Massilia in southern Gaul.
Arriving at the deserted Carthaginian camp, Scipio learned that Hannibal was three days' march away and decided to send his army to the Iberian peninsula under the command of his elder brother Gnaeus, while he himself returned to Northern Italy to organize the defences.
Gnaeus, with 20,000 infantry (2 Roman legions and 2 allied alae), 2,200 cavalry and 60 quinqueremes, sailed from Massilia and landed in eastern Spain at Emporion.
The Greek cities of Emporion and Tarraco welcomed the Romans, and Gnaeus began to win over the Iberian tribes north of the Ebro.
[6] In spring 217 BC Gnaeus commanded a fleet of 55 warships (probably quinqueremes) during a naval battle near the mouth of the Ebro River.
In early 215 BC the Romans, under the joined command of the brothers Gnaeus and Publius Scipio, crossed the Ebro River.
Hasdrubal tried to emulate his brother Hannibal's envelopment tactic (see: Cannae) but failed because the Roman cavalry held out and he could not close the trap.
In 212 BC, the Scipio brothers captured Castulo, a major mining town and the home of Hannibal's wife Imilce.
[9] With a large army at their back and observing that the Carthaginian commanders had deployed separately from each other the Scipio brothers decided to divide their forces.
[9] Leaving 2,000 soldiers in his camp under the legate Tiberius Fonteus, he marched out at night, to evade Masinissa's cavalry, and launched an attack on the Iberians in the early morning.
He caught Indibilis and his men by surprise and, with a numerical superiority, began to gain the upper hand in the ensuing action.
Mago gave the Numidians enough time to loot the dead before force marching the army towards Hasdrubal Barca's position.
Hasdrubal Barca decided to refuse battle and stayed within his fortified camp, he then managed to bribe the Celt-Iberian mercenaries to desert Gnaeus.
Their descendants in the male line continued until at least 46 BC, in the person of Metellus Scipio (who was adopted into the Caecilii Metelli family).