Yeah but I'm speaking about the AI doing it on it's own. I played Carthage, for example, and was therefore on the other side of the map. Pergamon is surrounded by the ''Independent Cities'' faction holding Rhodes (with which it was allied) and the ''Independent Dynasties'' holding Caria and Lydia (among others). When I put ''toggle fow'' on, Pergamon had attacked both of them, taking 3 regions, even the allied Rhodes.
That's because, as you say, Pergamon was "surrounded". The AI will always attack the weakest faction it shares a land border with, even if that faction is or seems stronger than itself. If there had been another faction close by, Pergamum would probably have gone after them.
I also don't like those vague and unhistorical terms as Gaul, Iberia or Germans, since they weren't unified states. I would much prefer something like Celtic Tribes, Iberian Tribes and German Tribes (it's not perfect but it's much better). In this way, we could, as The Sloth suggests, get rid of the superfaction Celtic Tribes and merge it with Free Tribes, opening a place for the Arverni.
I think we could rename the Germanic Tribes into "Suebi" without much trouble. As for the Free Tribes, we'll need to test it out first and see how the AI reacts to such a huge superfaction.
As for Gaul, my ideal setup would be the Aedui holding most of central Gaul, with all their major allied cities (Bibracte, Avaricum, Alesia, Lutetia, Lugdunum), and the Free Tribes holding Cisalpine Gaul, Gergovia and Belgium. That way, Rome can fight some good celts in Italy while the actual Gauls can continue to build up in France. I'd avoid a celtic civil war, as it seems to cripple both sides way too much (although we could try to have Aedui and Free Tribes start at war for the heck of it).
