Ok, ladies and gentlemen. I always use the same tactics against the gauls. I have never lost against the gauls while using my standard ass-kicking tactic.
Something like this:
Hastatii Hastatii Hastatii
Princeps Princeps Princeps
Velites Velites Velites
Triarii Triarii Triarii
Cav. General. Cav.
Hastatii: guard mode, shield wall and fire at will.
Princeps: shield wall
velites: look scary. rawr!
Triarii: look even scarier.
So all gauls charge straight at the hastatii, already getting heavy losses due to the pila. Then when all enemies are locked, set princeps and velites to fire-at-will. If the line is getting pushed back, send in the princeps. Once you send in the princeps the gauls usually break quickly. If not, the end you cavalry around the flanks and, voila! A beautiul massacre!
It always worked in the earlier versions, and they should work now aswell if there havent been any major changes to the A.I.