Celtic subgroup including Gaul. As far as most are concerned, Gauls are merely those Celts that lived in the region we call Gaul.
And it is not like CA ever said or tried to portray this in the game either: Gallic factions are merely those Celtic ones in the regions and provinces that represent historical Gaul. It's true that the lack of cultural differences within subgroups (ie those Celts in Spain, Galatia, Thrace and Gaul have no cultural aversion penalties simply because they are, according to the game, all in the same category), but that is more a matter of the game not having more minute details of that sort. In other words, your accusation is not limited to the Celts living on Noricum, but pretty much every other culture. The Etruscans are not of the "Latin" culture, and the Romans borrowed a great deal of Celtic and Greek designs in military technology and methods. So any aversion to different cultures were shallow anyways.
Ultiamtely the matter of a cltural aversion mechanic in the game not only serves a rather pointless purpose (it really is just another difficulty scale) but reveals how poorly designed the game was.
Ironically, you had Desert-Nomadic in Africa compare to the Punic ones by Carthage and its clients. Which despite being rather historically accurate, actually would not have meant much to real people back then. If anything, the Greeks and Romans would have been distinguished for their cultural aversion to everyone else.
In the end all I am saying is that this is not a particularly important issue. Not saying they shouldn't have better fleshed out unit rosters though, that problem is rife in the whole game. However I think this is also a case of narcissism of small differences