There is alot of myths about the gauls, or other barbarians at the time. They werent as uncivilized as one might think.
Or disorganized military.
Remember the greeks though of Persians the same way, if not worse... and we know they were everything but primitive.
To resume Gaulic armies, as a bunch of rable, naked gauls, its laughable at best as well. Imo.
They had better armor, and weapons then Greeks, or Romans, they were much more advanced technologicaly, in the manipulation of metals for instance, they invented, chain mail, and the use of montefortino helmets, wich the romans later on adopted for their legions.. i mean lets give the gauls a bit of credit here, there is a reason they were prized mercenaries as well.
Also the celts invented soap
Also i would recon, their armies, were much more mobile then greek phalanxes, and phalangytes, its not hard to imagine them outmaneuver this troops. They were also capable of forming shieldwalls when necessary, wich is basicaly a phalanx.
They deserve more credit, then they get imo historicaly. I mean the celts were able to expand troughout Europe and into anatolia, that means something.
I mean does this dudes look like it cant take on macedonian, or Greek troops?
Not all of them fought naked... in fact a selected few did...