They just need a few quick changes to have an improved historical accuracy:
aux barracks, celtic AOR:
lvl2:
swordsmen
lvl3: gaesatae
aux barracks, emporion, massalia:
lvl3: celto-hellenic hoplites
I'm trying to remember, I added some Celtic AoR stuff for Pontus I know, I think I just need to activate some for Carthage.
One problem I have is that I'm rubbish when it comes to modding units. I can add them if I have all the materials, but that's about it. So at this point, no swordsmen...
CV, the materials are available.
All you need to do is to get a slightly older version of XGM and get the galatian swordsman model.
You could even use the standard celtic swordsman, it wouldn't be perfect but still acceptable with the celtic skin.