The Brihentin are mounted noble cavalry, meant to break infantry lines and defeat lighter cavalry in counter charges.
Gallic Brihentin (Bree-hen-ten; "Knights") are heavy cavalry. They are used as shock cavalry to break an enemy position and destroy flanks, as well as overrun heavy infantry and slaughter lighter cavalry. Their spears, swords, and chain armor is all of finest quality, and their skill is great, though they are, as any cavalry, vulnerable to long spears and pikes. They can be used to best effect for riding down enemy heavy infantry and defeating light cavalry. Their armor can absorb many blows, and that should allow them to keep fighting.
Historically, the Gauls employed heavy cavalry as a method of breaking dense enemy formations, and were sometimes employed by Carthage, in addtion to various Gallic chiefs and kings. They were apparently of great enough skill to impress their employers; Hannibal grouped his Gallic heavy cavalry with his Iberian heavy cavalry during his campaign in the Italic peninsula. Such warriors would cost much to outfit, but they would surely be worth everything spent to ensure they had the best possible equipment.
These guys are not comparable to the elite cavalry of more eastern factions but are one of the heaviest cavalry units in the west (the heaviest in the west being the Iberi Lanceari). They are able to carry out an effective charge and hold their own in a melee fight with their Celtic longswords, while well armoured in chain mail and a Montefortino helmet.