Fire! And lots of it!
Seriously though, don't attack Gaul until you have at least 1 1/2 stacks plus a decent garrison in each city. I like to take that first one (the one you get a your first Senate mission) and then go take Corsica/Sardinia from Carthage and then call a Ceasefire w/ Trade. THose 4 cities are nice enough for a while. Then I wait until one of my two starting cities gets big enough to train Principes and once that happens I start pumping them out along with Hastati and maybe some cavalry if I can (but just for support/mopping up routing units --> this is a must IMO because the Gauls usually rout completely and when their units still have 50%+ of their soldiers left so with 2-4 units of cav, you can easily wipe out routing Gallic armies).
In battle, use the "checkerboard" formation mentioned above as it will keep their generally superior numbers from getting around your line if it's just a single line. When they get close enough, turn on auto-fire and shower them with pila. I've broken entire units of swordsmen/warbands just because 2 units of hastati/principes hit them with a concentrated volley of spears.
Seize Mediolanium as soon as you clear your side of the Alpines of their armies (easily routed and destroyed as per above). Garrison it tough to hold back reinforcements, while a second army of at least 1/2 stack takes Patavium either by assault or starving them out if you're low on man-power. Once you drive them out of your side of the Alps, they will find it impossible to gain a foothold if you hold the pass to the west and the north-west (NW one is done best with decent garrison at Mediolanium). From there I usually like to sail over and take Masilla as a strong fortress city to the west of the Alps.
From this position (generally as fast as you can without risking your armies, ie. don't dawdle a few turns away by waiting to see if they are sending men to your side of the Alps, because most of the time, they are
) you can begin really taking it to them as they will have lost two decent cities and by then will probably be at war with Iberia, Britons and Germans, allowing you to make powerful alliances with any of those tribes
Heres how it went:
Gaul did indeed make the first move, and not long after I started the game, too. They had attacked one of 2 of my forts I placed at the river crossings just north of my beginning cities (they attacked the left one, closest to Gaul). So, I was all "[censored]!" and decided not to stand about, even with the mega lack of man power I had.
I took Mediolanum while I had the chance, despite the huge army they had standing about; I guess guarding the western pass through the alps (I guess it was a mess up in the AI). So, after I took Mediolanum, they marched that huge army just south of the river, north east of Segesta. By that time I had an army to challenge them, and when the odds are even, I win. After completely destroying that army, they called for a ceasefire, which i obliged, but not before taking Patavia and demanding an 800 gold tribute for 8 turns (after much talking down price wise). After that, they decided to bulk up and move their armies west to Spain.
While they were bulking up, I took Massilia, put the head of my standing forces there, and advanced my economic and military technology. After some time, they declared war on Spain, my ally, as I figured they would. I jumped on the opportunity take their not-manned-enough cities of Lugdunum and Narbo Martius. For some reason, they continued the war in Spain, keeping their 2 full stacks in that area (and still do at this point in the game). So, after solidifying my claims in those two cities, I marched north to their capitol of Alesia. I have yet to play more.
I totally have the smart option of starving Alesia out, but despite their sizable force of experienced heavy infantry, I'm going to march in and cut them down for all the trouble they gave me. If I could burn down their main hall and raise a tall Roman flag I totally would.