Neitos are very disciplined heavy infantry fighting in a dense formation.
Gallic armies of the late period relied upon the same shock tactics they had employed during earlier times, but the need to combat increasingly better armoured and trained enemies led to the formation of a professional force. Well armored in quality chain, with sturdy shields, and good longswords, the Neitos (Nee-yet-os; "Soldiers") are professional soldiers, and excellently skilled and disciplined when compared to the bulk of light Gallic soldiers. Their charge has great strength, and their skill in close is frightening; they would be capable of fighting hardest where needed.
Historically, Gauls began to move more toward a better armored, more professional army in their twilight. Enemies beset them on all sides, and the desperate need for a better equipped force grew blatantly apparent. While similar soldiers had existed for decades, they were rarely in large enough number to act as anything but a tiny elite, though they, in truth, were little more than average soldiers of regular warriors in superior armor and with superior equipment. Ironically, most of these soldiers, meant to face up to outside threats, met their ends against other Gauls, rather than their foreign enemies in the last Gallic war, when Gaul was incorporated into Rome. Gallic auxilia and allies, of both Rome and Carthage, were actually employed in this manner before such soldiers were used widely by the Gauls themselves. The Romans did this to ensure their Gallic allies and auxilia had an edge over the Gallic warriors they engaged, and Carthaginians re-outfitted their Boii allies after Cannae in this manner.
This is a mail-armoured variant of the earlier Gallic swordsmen types, with a smaller unit size, but better morale and overall stats. Neitos are mercilessly efficient heavy infantry, with highly lethal swords and a decent resistance to missiles. As such, they are obviously important troops for the Gallic factions during late campaign, when you will have to face the javelin-happy Iberians (especially the Lusotannan faction), the Roman war machine, the tenacious Germanic warriors, and possibly some Hellenistic warrior types. Neitos are well equipped to deal with a variety of enemies. With them, defeating Lusitanians and Swȇbōz becomes much easier, and holding off Legionaries becomes less of a problem, too.
The main use of Neitos is in the battle line or as assault troops. While they can hold their ground against cavalry, it is better to use them as dedicated anti-infantry forces and leave the cavalry-killing to the Gaelaiche (Celtic Spearmen).