I don't think you'll find an author that says "Legio X fought Uirodusios". AFAIK as I know they're a representation of the greater mass of Celtic warriors that fought naked, not all Celtic Warriors that fought naked = Gaesatae.

Uirodusios from the description = Man-Demons a stylized name I believe to portray their psychological impact of fighting naked + the rage they worked themselves in for the battle.