Providing you're playing on normal difficulty, the Gauls are, to be blunt, crap.

As the Julii, get a full stack of Hastati, with a general and a couple of equites units. Stretch the Hastati out in a long line three men deep. Put them all on fire at will, turning it off whenrever you need to prevent friendly fire from firing into melee. Place the general in the centre behind the line, and one unit of equites at each end of the line, set slightly back from it.

If you have velites in your stack you can place them in a line two-deep in front of the hastati and withdraw them prior to melee. Personally I don't really see the point of fielding velites against the Gauls, as they just get in the way and often cause friendly fire.

On the attack, advance towards the enemy line until within pila range, allow the hastati to fire all their pila and then double-click charge across the gap into the warbands or swordsmen.

On the defence, allow them to come to you. Loose off as many pila as you can while they attack, but allow enough time to countercharge the Gauls. If they charge with their general, stand and receive the charge with the Hastati, then charge your general directly into the melee at him, while also flanking with the neighbouring Hastati units if that doesn't expose them to flanking. His general will probably die during your general's charge.

Once melee has started the value of Roman morale becomes apparent. The thin lines of superior Roman troops in your centre can easily fight for long enough for the ends of your line to turn the Gallic flanks and for the equites to gallop into the enemy's rear, routing any skirmisher warbands (which the Gauls rarely use anyway) and then turning to smash the centre of the line. If the Gallic army is lead by a non-cavalry captain, then he will be standing behind the main line, and the equites can be used to kill him off.

The whole Gallic army will soon be routing, if it isn't already, and your cavalry units are well-placed to run down the routers. If you're lucky enough to have brought along some war dogs, these should be sent against the very first unit to rout, after which they'll keep attacking until the enemy has left the map.

These tactics should get you 1000 Gallic casualties for about 50 of your own in a large battle.

When assaulting Gallic cities, create several breaches, send hastati through one, then use the other to charge their rear, probably with cavalry. If you're happy exploiting the ai staying static in the town square, line your hastati up in a ring around the enemy and use fire at will to break them down with missile fire. Velites or that starting unit of archers can come in handy at this point. Once all the ammo has run out, charge in to finish them off.

Oh, and once you're rid of the Gaul's, beware the Germans. They're not scary, but their specialist troops are much better than those of the Gauls, and their phalanxes gave me a nasty surprise the first time I met them.