Actually if you look at the history of the early republic, you'll notice the Gauls win more then once; according to legend, they had to on one occasion be saved by Juno's sacred geese (fittingly, the in-game geese are more useful then the historically nonsensical war dogs). Part of the reason for this is that unlike Marian and imperial armies, republican legions weren't particularly well equipped or trained; before the manipular system, they weren't particularly well organized either.
Aside from having access to somewhat better resources due to trade, the Greeks and Romans had no intrinsic advantage over the Gauls when it came to weapons or armor manufacture. In fact, we know that certain military technologies like ring mail were originally developed by Gauls and made their way to more "civilized" land. Obviously these Gauls aren't out of their depth.
The civilized lands also had no inherent advantage when it came to military organization, which militia armies are notorious for lacking in. At least the Romans had the good sense to steal any good idea they came across, like the manipular system, but the Greeks were often too proud to do the same.
I'm tempted to say that the Gauls were also more warlike, and had that inherent advantage, but I'm not at all sure that's true. Greek mercenaries were highly sought after in the classical period. Of course that might not have helped the city states themselves if those mercenaries were away in some foreign land rather then slouching about in Greece waiting for a war to break out.
One way or the other, I'm not trying to say the Gauls were somehow inherently better at war then the Greeks. What I'm trying to say, is that they weren't inherently disadvantaged either; they're not unwashed savages (well, no more unwashed then was the norm before indoor plumbing).
Urbanization is overrated when it comes to pre-industrial warfare.