Alright, Ram! Good to see you in the right place.
Well, actually I'm one of the few big fans of the Gallic Warrior. The GW is a very underrated UU. Personally, I like them better than Praets on higher levels.
The key to GW is twofold. First, it is basically the only unit outside mounted units that provide a tactical advantage on higher levels. Yeah, Impis can use tactics but are really not strong attack units above...say..prince. The fact that GW start with Guerrilla I (only melee unit to get this promo) and can start out the gate with GII, allows for faster movement across the map using hills. Make use of hills around enemy cities to attack fast and surprise.
Second, the G line on GWs and the fact that both Boudi and Bren are Charismatic, means that you will get GIII very quickly, and even out the gate with a settled GG. GIII is very powerful. 50% retreat odds is very nice on a strong early attack unit. When you start adding City Raider on top of that, it becomes crazy. So basically, GWs are very strong before siege arrives. In addition, the defense bonus for GII means you don't need as much in the way of support units.
If you happen to build a Dun - an generally weak UB - and have Archery, you can send along some GII-III archers with your stack for some additional protection of your GWs and those ARchers can keep pace over mountains. So some synergy there.
Also, note that although you do need IW for GWs, you can build them with either Copper or Iron.
Boudi is obviously a great warmonger leader with Agg/Char as the GWs start with CombatI/GII out the gate, but Bren is good enough with Char and Spiritual is obviously a much better trait than Agg. Plus, as I'm sure you know, it is good for culture wins.
The main issue with Celts is poor starting techs and really no Economic traits. CHAR is a good trait, but you really have nothing to boost your way to IW. Ideally you will get a start with some commerce resource nearby to speed things along or get POT asap to get some cottages going.
Main thing though is to get GWs online quickly, keep building them, and unleash your Celtic wrath on your neighbors. Use that G line of promos.
As for Bren, I do know that as an AI he is often generally disliked early by many AIs, kinda like DeGaulle. This may apply to a human playing Bren with hidden diplo maluses with the AI, which may put you at a diplo disadvantage early. Always note your diplo situation before and during an attack on an AI, especially after Alpha is in play. If possible, try to get peace treaties, but otherwise make sure you protect yourself against potential aggressors.