The A.I. gives priority to attacking the player above any other factions, so when you are playing a central faction like Gaul, you will always be at war with at least two factions. Personally, I would focus on establishing easily defendable borders with the Britons and the Germans. In your case, this probably means taking the remaining Briton towns. Germania is poor however, so it is not a good target, unless you can take out the Germans fast. A better option would be to establish a defendable front line and set up spies/watchtowers to preempt German attacks.

The Italian peninsula is rich, so the current owners can afford to send stack after stack of high-quality troops at you. Getting into a war of attrition with them is a bad idea. You need to take them out quickly. Don't be afraid of the exterminate button when taking their cities: you may not be able to spare the garrisons, and large populations are a bad idea for the barbarians anyway. From what I remember, Gaul foresters were not that good. I would recommend teching up to Noble Cavalry: before the Marian reforms the Romans don't have a proper counter to this unit, apart from their bodyguards.