Please register for Total War Access to use the forums. If you're an existing user, your forum details will be merged with Total War Access if you register with the same email or username. For more information please read our FAQ’s here.
Unless the unit is supposed to come from outside the main military chain, I don't see any building level that unlocks recruitment of the Gallic Axemen unit for the Insubres. It does get used as a garrison unit and one of the starting armies has one included, but I don't see anyway to recruit the unit in the campaign.