You're not on the highest difficulty level I hope?
I have to admit that it's quite some time ago that I tried the Julii. So I did some checking to see if my initial thought was right... The first thing you have to do is to take the two free cities, Segesta and Massilia. Starve Segesta (sieges are boring and the city siege pathfinding is enraging anyway), then rush to Massilia. As a basic pointer, go for any harbor city you can get, since that's where you get the most commerce from.
If you have taken Segesta you can build some forts and then put a token army in it. The zone of control will prevent Gaul movements.
If you have taken Massilia, you've essentially locked the northern italian cities in. This will prevent the Gauls to send in reinforcements. Also: if you just let them do whatever they want, they'd attack you anyway. So you really want to take Massilia to hamper their movement. Before you attack the Gauls you have to build a decent army, something like 8 Hastati, 4 archers, 4 velites, and 8 Principes. Take some cavalry/equites, but don't bother too much with them. If you can't get the principes yet, go with more hastati instead. And unless you're actually trying to get a city through city combat, it's probably smart to put your units into two seperate armies. Don't keep them far apart though.