I don't think it would have differed that much, except (maybe) the urban Guals being poorer wrestlers (which is handy in single combat but not in this game). I do think, however, that the standard unit would (and should) simply be some kind of spear user. Now, for the sake of the game they don't have to be SPEARMEN class units (unless giving them a spear always does that). They could be light or heavy infantry just modeled using spears, given some kind of nasty charge ability to represent them being spear users but agressive and disorganized (i.e. they don't get the bonus against cavalry for being spearmen).
Also, generally speaking "peasants" don't fight, especially in a warrior culture. Remember how to join the Roman army (pre-Marian and before) you had to be a rich Roman landowner? Well the Gauls and Celts and Germans were that way too. Gaully McScratchEarth isn't going to become a spear-wielding warrior when war season is on. Instead, it'll be land owners, their family and perhaps a few select others that march to war because they can afford equipment AND they have time to learn how to fight to a higher degree than everyone else. If they didn't, they couldn't hold power -- if all your peasants are armed, they now have political power. With slavery, however, I do wonder what proportion of their population was "peasant". It's...a complicated topic.
Swords require a lot of iron. Iron that is hand mined, crushed, roasted, bloomed, forged and sharpened. This quantity of iron has to compete against other uses: Tools and other weapons, as well as export.
Also, Quinn, this might be handy:
http://classics.mit.edu/Caesar/gallic.1.1.html
Words of Caesar himself.
M.