I've only played this faction on quick campaign med/med, but I had some success fairly early.

Immediately on starting the campaign you will note that the Gallic lands are split in two, two provinces in Italy and the rest on the other side of the Alps. First priority is to join these two together.

I raised armies in Alesia and Lemonum and sent them south to deal with the rebellious provinces there. Make sure that the army that will be heading through Lugdunum to Massila has a character with it, as you will need to build some watchtowers once you have taken Massila. Keep churning out units from Alesia to reinforce your armies and free your attack force from garrision duty.

Whilst this is happening send a diplomat to forge an alliance with Germany, and trade rights with Britain (perfidious Albion will always stab you in the back in the end, so I don't bother negotiating an alliance). Also reinforce Condate Redonum so that you have a force ready to attack if/when the Brits declare war. I also reinforced Mediolanum and Patavium, as the Julii are bound to attack you sooner rather than later.

I ignore Spain, except to beef up the Numantia garrison (the Spaniards kept on trying to bribe the city in my campaign), and negotiate trade rights with Spain and Carthage.

Before long the Julii will attack. Your newly conquered southern provinces should be bringing in a bit of money by then. I used Patavium as my military base, and sent an army from there to attack first Segesta and then Arretium. I use Massila and Mediolanum to supply Patavium with fresh troops. You have to watch out for the Brutii as well, so I sent armies to take Illyria and Dalmatia from them, and this kept them off my back whilst I was taking apart the Julii.

As sure as night follows day, perfidious Albion will attack you as you are fighting the Julii. Use a combination of diplomats and your army in Condate Redonum to push them across the Channel. I then raised a fleet using Numantia and Condate Redonum and used this to ferry my army across to Londinium. Once you take this city, the fight seems to go out of them. I ferried a garrison over to Londinium and used my conquering army to attack first Eburacum and then Deva. I didn't bother with Ireland.

Back to Italy. By now SPQR will have taken an interest in you. I first attacked Ariminium to finish off the Julii, and sent diplomats to bribe away Scipii and Brutii reinforcements. Then comes the attack on Rome. This is really tough, because the Senate have a very powerful army stationed near the city. I sent a full stack from Ariminium to attack the SPQR army which had moved outside the city. I managed to batter their army, but took severe casualties, so I retired to Ariminium, retrained, and then attacked again. Oh, I also blockaded Rome with a fleet I had built in Massilia. Second attack was succesful, so I sacked Rome. At this point I had my 15 provinces, so that was the end of the short campaign. Had I carried on, I would have finished off Italy, and then probably attacked Spain.

A note about the Gallic armies. These troops are utterly rubbish. You won't have much trouble fighting other barbarians, although British chariots can be a bit nasty, but the Romans are a different matter. I use as much cavalry as I can, and use warbands and swordsmen to pin the Roman troops whilst these guys sweep around the flanks. I also make a lot of use of wardogs - they are great for chasing those annoying velites. Whatever you do, you will take more casualties than you might be used to playing a Greek or Roman army. This is why you should set up a unit conveyor belt from Transalpine Gaul via Mediolanum and Patavium to the front. Because you are fighting the Julii and then the Senate almost from the start, you won't have all that many good troops. I did use Druids in one battle, and they were impressive, but realistically you should expect to be fighting with warbands, swordsmen, barbarian cavalry and dogs. Having a Temple of Tertullus (sp?) and a blacksmith in your troop producing towns will help.