The "Ugly Women" thread reminded me of something. Tacitus and Ammianus Marcellinus both mention Celtic noblewomen leading armies and participating in battles. I also seem to recall a diplomatic dispute, some time between Caesar's initial landing in Britain and the final Roman conquest when the Romans were playing their usual game of divide and conquer with the Briton nobility, when their aribitrator stripped a female noblewoman of her title to rule a small kingdom in southern Britannia because they did not recognize a female inheritor to the throne.
I've not seen any evidence to indicate that Celtic women-warriors were widespread enough to warrant dedicated units, but it would be kosher for one or two of the Gallic characters to be female rather than male. I can't imagine it would require any change of game mechanics either, depending on how the base game's gender tags work. Hardly a crucial point, but the option for male player characters to marry into the Celtic nobility the same way female player characters could would be an interesting way to add extra differentiation to the Gallic faction.