Before the Romans met the Germans the big bad wolf of their history was the Gauls.  If the German raiding along the Rhine was bad, the Gauls had topped that by some distance by sacking Rome itself in 390 BC, or thereabout, when Brennus, the Gallic leader, coined the term ‘Vae Victis’.

In their hatred of the Gauls, the Romans went out of their way to present them as barbaric, but in truth Gallic society was an advanced modern civilisation with large urban centres, advanced metalworking skills and a semi-feudal nobility as well as a social infrastructure which would have been recognisable in Europe a thousand years later.
The social hierarchy of Gallic society meant that a class of Noble warriors displayed all the trappings of their wealth.  High quality armour and weapons as well as often fighting mounted on horses with their stewards accompanying them to provide remounts and additional weapons.  This gives the Gauls a speedy, hard-hitting core to their force that, whilst somewhat brittle due to small Group sizes, is both capable of snapping up any Roman Groups that over-extend themselves as well as delivering an effective attack on the toughest troops when required.  Don’t look on the requirement for cavalry to withdraw after one or two rounds of combat as a negative; it is actually a self-preservation mechanism there to protect them and allow them to keep returning to the fray afresh.
Like all Barbarians, the Gauls need to play to their strengths.  The terrain there are fighting in is likely to be more open than the Germans, but that suits their noble cavalry.  As a result, ambushes are less of a primary weapon but should not be ignored as they can still provide a very effective weapon.
Fervour is clearly still the key to defeating the enemy.  As we have seen, the ability to attack and have the first four, five or six points of Shock inflicted on you cancelled out by Fervour can be a huge advantage, especially if you can cause a similar number of points of Shock on your opponent.  It is well worth reading all if the tactical guides as the more general comments regarding the Britons and Germans will largely apply to the Gauls.  Check out the Romans too: know your enemy!
Selecting Your Force
The Gauls have probably got one of the nicest choice of support option available to them.  Due to their wealthy culture they can hire mercenaries, either British or Germans, but their force compositions give them a quite specific choices to make, namely do they want to fight primarily as a mounted or foot force.  If they prefer mounted, then the ability to choose either Noble or tribal cavalry will give them the ability to create a highly mobile force that can swirl about the battlefield, rushing in and attacking with javelins or attacking and fighting before disengaging to rally back, ideally onto waiting stewards.  This type of battle is difficult for the Romans as they are unlikely to have sufficient mounted troops to counter this approach.  However, brittle as cavalry are, be aware that one bad round of combat can see a cavalry Group much reduced in effectiveness.
The alternative, choosing a predominantly foot force to do the heavy lifting, still gives you lots of choices.  Don’t forget the Fanatical warriors who are great at breaking the cohesion of a Roman force and if you want more warriors consider the Ambaxtoi as their medium armour and elite status makes them a very tough opponent. If you need to sacrifice points elsewhere the Woodman archers are a good choice as long as you have some good terrain to stick them in.
Deployment Options
When deploying onto the table, the Gauls should consider the terrain.   As with the Germans, refusing to deploy until your opponent is deep into your territory will allow the enemy time to make errors and straggling units to emerge due to the card activation system.  However, with a mounted force deploying early and using your javelins to harass the enemy, even those of your Mounted Warriors, will hopefully frustrate the enemy, obliging them to use Command Initiative to rally off Shock.  One key point here.  A Roman Centurion with three Command Initiatives can advance very effectively, beginning his move in Close Order, opening out into Open Order, moving swiftly as a result and then changing back into Close Order at the end of his movement.  He cannot do that without burning Signa Cards if he has to rally off even a single point of Shock.  Harassing the enemy is well worth it even if the victories gained from it appear small.
Choose your moment of deployment carefully, whether with foot or mounted troops.  As with the Germans, try to snap up small, isolated enemy Groups, especially if they try to shut down Ambush Points and are isolated when doing so.  If you are using a force with ample cavalry you’ll probably want to decide how to use the combination to best effect.   A cavalry fight before launching your foot troops into the fray can be good as it allows the mounted troops time to rally off Shock before going in again. That said, keeping your cavalry bac to launch a coup de grace can be very effective, especially if your main foot force is the tough Ambaxtoi who are likely to leave the Romans chewed up after their encounter.
Before Contact
As with your German neighbours, remember that controlled movement is best used up to the moment that you send the attack in.  Whilst you can stream across the table very rapidly in an Uncontrolled Mob, if the enemy attacks and hits your Mob piecemeal, rather than as a body where the Groups can offer mutual support, you will be in all sorts of trouble.  Reforming a Mob that has moved in an Uncontrolled manner is not quick or simple.  Your Leader will need the Leader to issue a ‘To Me!” Command and that takes time and saps Fervour.  Moving as a Controlled Mob should still be rapid if you have built up enough Fervour, so save Uncontrolled for the final charge to contact when the ability to ‘change shape’ as you move will hopefully give you some tactical choices.
As always, missile troops are a good way to chip away at the Romans.  In Gaul you’re likely to have space to give these troops a good field of fire so deploy them early and focus on the Legion if they are present.  As we have seen before, Shock is relatively easily rallied by the Romans, but if your missile troops can oblige them to burn a Signa Card or two this will reduce their tactical nimbleness, even if only temporarily.  Using your missile troops to get the Romans to put their shields up just before making an attack with Warriors will at least burn a Signa Card.  Seek out small tactical wins like this to sap your opponent’s command and control superiority.
In Combat
Fervour is, as always, a major advantage at the start of any Close Combat, but it is fleeting and burns out fairly rapidly.  Once Fervour evaporates off, your Groups are at a huge disadvantage due to their lack of armour, so don’t be tempted to slug it out.  Push up fresh Groups early and then use the Break Contact order to disengage and rally back as soon as Fervour disappears.  As soon as you disengage, this is when to throw in another attack.  With Noble cavalry this can be fast and hard hitting, so try to keep your forces in a position where they can mutually support each other.  With the Roman ability to strip off Shock relatively quickly, it is critical that any Barbarian force gives them no respite but keep constant pressure on the enemy.
In summary, the Gauls must harass the enemy to encourage them to make errors and over-commit their troops.  If the enemy allow Groups to come loose from their main body they can be snapped up and disposed of.  Cavalry are extremely good at this as their hit and run approach allows them not only to get into contact but get out again before any enemy reinforcements can arrive.  Ultimately, taking on the enemy’s main body will demand a coordinated approach where two or more blows by fresh troops are launched in waves.