I can try to discuss the equipment of the Gallic and Romans. With the Marian reforms, the Estrucan/Greek/Roman mixed helms such as the Estrucan Corinthian used mainly by triarii become more rare, and the Attic became the helm of officers and the Montefortino become the helm that most legionaries used. I have a picture of the helm, and a model that I prepared for IG as a kind of easter egg item(I also said I'll let Darwin use it).The helm, was originally a Gallic helm and the Romans eventually adopted it after getting invaded by the Gauls.
[img]http://farm4.static.flickr.com/3092/2592280706_18de0b6ca4.jpg?v=0[/img]
Other then the Montefortino and its variations, the barbarians enjoyed the use of the Agen, which was the ancestor to the Imperial Gallic Helm.
Another consequence of fighting with Gaul was the increasing reliance on mail armor, with bronze square plates being phased out by the time of the Marian reforms. As for the Gauls, only their elite warriors utilized mail armor, and if this is to be a historical game, great care must be contributed in not confusing Gallic mail shoulder reinforcements from Greek style reinforcements, which the Romans used.