I have to ask...if you know nothing about these peoples or the history of the period why are you insistent on making a historically accurate game based on this period? Just on a personal note I think you should try to either focus on something you know about or make a real concerted effort to know about these things before you make this kind of choice. I know that's precisely what this post is trying to achieve but, y'know?

I suggest you look at our extensive book list in this area and see what interests you. If you're especially keen on the Gallic Wars you may as well read the work from the man himself, a personal commentary on the matter by Julius Caesar.