Personally, I find it difficult to fight Romans as Gauls without axemen or other AP infantry. Reason being, Triarii (especially Camillan era), Pedites Extraordinarii, and Polybian Principes are very difficult to kill if all you have is spearmen and swordsmen. Sure, you have slingers and lancers for an AP punch, but they can't slug it out in melee. And slingers are notoriously bad at killing 4+ shield units from the front, and difficult to manoeuver around on the battlefield. Plus it's lame to spam those units. While Bataroas are the main killer unit in Gallic armies, you should always bring some Appea Gaedotos or Teceitos, or even Tekastos or Asturian axemen when fighting Romans.