The lack of decent heavy cavalry makes the Gauls almost unplayable in MP. I would never pick them on a flat map, any half-decent general with a cav heavy army will rout you out of the map within seconds. The foresters are too costly compared to ther elite archers, and chosen swordsmen are inferiour to the elite roman inf, what is worst they will just collapse under a mass cav charge.

Your only hope with the Gauls is to fight in forest. First, most of your units get a bonus; second cavs -your greatest fear- are less efficient in forest; third, you can hide your units and play a cat-and-mouse game. This is your only chance: a guerilla warfare; in pitched battle, especially in the open, you are doomed to fail.

As for the army composition, you have only two decent units: foresters and chosen swordsmen. Both of them quite expensive so you need imho at least 15000 denarii to make a decent army. I would bring a few noble cavs (3 or 4) but stricktly for harassing -so dont spend too much upgrades on them (since they wont hold against heavy cavs anyway). One or two other units -such as naked fanatics or dogs- can spice up things but dont expect too much from them.

Good luck! If you can win the with the Gauls, that should give you a great confidence! Mastering other factions will be a piece of cake ~:D