Playing with Gauls is being a very interesting experience because, so far, they seem to be the faction with the worst manpower pool I've seen so far (maybe Numidia is worse, without Conquered Tribe and Warhorse Herds). A 4 on skirmisher cavalry (which is nice, until the first reform, then it falls to a 3), 3 on mid tier tribal spearmen infantry, 3 on an okay skirmisher (2 after reform), 3 on a tribal levy that is pretty handy at eating charges, missiles, and tiring enemies... that's okay. Problem is, that is my capital, even on the top government. The Allied City government that is pretty prevalent is worse, offering mostly 2s, exception being the levy spearmen, and migrations being a similar issue. Allied City, furthermore, can't be upgraded to Confederation (if I read it correctly?), so turning a settlement into an Allied City that can be later turned into a Confederation is a major waste of resources and time.

So, regarding manpower, I personally would put it somewhere bellow Rome (great capital that can make an army by itself, meh Socii without colonies that can just contribute at most), probably bellow the Pritanoi as well (that got good recruitment of low tiers with Tribal Community and decent of mid tiers and cavalry with Large Tribal Communities, no idea about petty kingdoms yet).

So, how I have been navigating around it? Luckily Celts in general have fantastic battlefield surgery on the Druid traitline (although these ones clearly need it more than the others...), so this helps to keep my losses at low and make the most out of my men, but furthermore, something that I rarely did before, I have been scrapping out the bottom of the barrel of the mercenary pool, to some success.

Gaul (well, Western, as I am playing Arverni for now, but not feeling too much like trying the Aedui as they seem to be pretty much the same experience) got quite a few Mercenary regions, considering its size. With my territory (pretty much everything west of the Aedui, plus some advances I managed to make up to Normandy, north of the Pyrenees, up to almost Marseilles), which corresponded to roughly 8 provinces, I had access to at least 3 regions. Recruiting only the generic Gaul mercs (because of their interchangeability with the rest of the regions, easing up reinforcing in possible fronts) + the mid tiers of my capital + my plentiful skirmisher cav I could make some fairly decent armies real quick, although not very cheap. The problem with relying on the merc pool is that there's nothing you can do to reduce their recruitment cost unlike normal recruitment, but it did allow me to set up Protectorates everywhere to boost my economy quite nicely. Problem is, as I am at war with the Romans, things are starting to get a bit too tight for my liking, specially as the Pitanoi keep attacking me (I can always make them pay for peace immediately after though, but still, I am afraid it is a matter of time before I have to face them), and the Celitberians might go north any moment as well. And the Aedui are being friendly so far, but I am playing on Hard, so I don't know when I might get stabbed in the back.

So, is this how Gauls are meant to be played? Relying mostly on mercs? Do the later reforms improve my manpower pools? Am I missing something? And are the Boii just objectively superior because of the colonies, as the top government, military-wise, seems quite underwhelming? 4 recruitment slots with the same manpower as the confederation. It does make a nice gathering point for retraining my men from elsewhere, though, as well with the +1 experience it provides.