I just completed CIG as the gauls and am now playing as the romans. I found it a little bit of a struggle at first, then easy as pie as soon as I had a solid base. Throughout my entire gaul campaign the romans never declared war on me. It wasn't untill after I'd wiped out every other faction on the map that I turned to them, and even with the whole 'military escelation' thing I wiped them out without breaking a sweat. The trick is confederation. Every time I wanted land and such I would offer the owner confederation,. he would decline, I would declare war, lay siege to a town or destroy an army in the field then offer again. They almost always accept, and usually give me money on top. Even the belgians took my offer after I razed a couple of towns. I ended up with so many statesman from disbanding the stacks I got for confederation I never had to pay for a general once. The hardest fights were against Suebi and Massila, Suebi because the geography of germania is crippling and Massilla because they built up four stacks on an upgraded province capitol and refused to move.

I'll see how it goes as rome, but the key there will be divide and conquer, make frinds of gaulic tribes and get them fighting against each other, then wipe out the ones your not friends with one by one. My biggest difficulty so far with rome was taking a big enough province to build a military production base.