I think the problem I have with the UU is that a) for a swordsmen of any type - UU or otherwise - to be effective against cities, it needs to go down the City Raider path.
An unpromoted swordsman is a better city attacker than a city raider I axeman (6.6 vs. 6). A city raider I swordsman is a better city attacker than a city raider II axeman (7.8 vs. 7.25). A city raider II swordsman is a better city attacker than a city raider III axeman (9.3 vs. 8.75)*.
This means you can afford to give Gallic swordsmen guerilla II and they will still be better city raiders than axemen. The speed boost of guerilla II is very often a good enough bonus in itself. You'd need bad luck with hill placement for it not to be.
As for guerilla III. An axeman with two more city raiding promotions than a swordsman is a better city attacker. Before 2.08 I considered it a niche trait. I did not agree with the idea that Gallic swordsmen should be given this promotion unless circumstances (attacking hilled cities) required it.
Post 2.08, sending the majority of your Gallics all the way down the list of guerilla promotions is a useful strategy. The 30% withdrawal bonus applies to all terrain (I give it to my crossbows - particularly cho-ko-nus - as well). While the withdrawal chance doesn't improve your Gallics city raiding abilities it does improve their chances of surviving all attacks and increases their chances of reaching higher experience levels (especially since Brennus is charismatic). For me, this is quite often worth the detour from the city raiding promotions.
* Assuming melee units aren't defending.
Edit:
*Now, pop rush the Dun (50 hammers - 2 pop). Then pop rush an Archer (or chop with the captured workers). Forget about the Barracks: An Archer with Guerilla I on a hills defending a city with the Dun is more powerful than an Archer with City Garrison I (from the Barracks), Guerilla I (Barracks) or even Guerilla I and 2 (without the Dun).
This is the only part of your strategy that I'd disagree with. Pop rushing walls is fine but I'd have one of my core cities pumping out archers with 3/4 XP and guerilla II and sending them to defend your newly conquered cities, leaving the newly conquered city to work on improving commerce or culture. Alternatively, if a hilled city needs defending, have the core city producing Gallics with guerilla II to be your city defenders (a task you already know they're good at). They can also double up as emergency attackers.