If I may ask, from where are you getting the idea that Roman legionaries fought in a manner similar to modern riot police? The only place I've seen riot police being referenced is by Krentz in Men of Bronze, in which he references riot police tactics in arguing for a more fluid and inclusive interpretation of fifth century Greek hoplite formations. Critically, Krentz uses the formations of riot police as evidence but does not in any way say they offer a model of hoplite warfare. While the methods of riot police may assist in interpreting certain aspects of Roman warfare, I would be skeptical of an author who claimed they provide a lens through which to view the mechanics of Roman battle.

Ok well here we go...

The Gauls fought similarly to many of the other so called "barbarians" of the Roman world. The reason for this similarity is because of the structure of their society. Gauls and other relatively less developed groups were not as centralized as the Romans and identified heavily with their families and tribes. In this sense they would often compete with their neighbors and warfare was largely based around raiding. In fact, small scale raiding was rather endemic and had even been a feature of early Roman warfare.

Historians like Adrian Goldsworthy point out that for the Germans there was no real justification needed to raid other than the weakness of your neighbor. I think it's fair to extend this comment to the Gauls who fought similarly. We therefore see large amounts of opportunism. The leaders most able to effectively capitalize on such opportunities would gain prominence in their tribe. Powerful warlords would gain attendants who would follow them into battle. The more loot and glory such leaders gained the more influence and attendants they received. It would be possible for them to eventually dominate more than just their tribe but this would rarely be longlived.

The nature of Gallic armies therefore reflects this pattern of warfare. The core of a force would be drawn from a warlord and his retainers. These would usually be more heavily armed and armoured because of the accumulated wealth and loot from their raiding successes. The rest of the army would not be so heavily outfitted. The rank and file of a Gallic army would probably be a mix of spear, sword, and javelin armed troops. Gallic cavalry was also present and had a fine reputation.

Arms and armour of the Gallic forces would only improve over time as Gallic civilization developed. We can also note that the Gauls are were actually rather well known for their metal working and could produce fine ring mailed cuirasses that the Romans and others adopted.

In battles the Gauls would form into large groups focused around various warbands. They relied on these elite forces to lead the charge and cut through enemy lines. Gauls very much depended on breaking the enemy in a head on crush.

So as a recap for the Gallic force we find that they had very competent elite troops who had gone through a lifetime of fighting. The common soldier still benefitted from the raiding lifestyle through the cultural expectations of ferocity and bravery in battle. On top of this they could also sport good quality cavalry.

When we look at the Romans it really depends what time period you want to analyze them in. On the whole though we can say that they were not super soldiers and as individuals they were not naturally superior fighters. In fact the transition away from relying on farmer levies from the early legions to the poorer recruits following the Marian reforms would have decreased the average recruit's physical qualities and martial prowess. This can be contrasted with the Gauls who still drew their men from a people accustomed to warfare and physical labor. Additionally Rome traditionally fielded very poor cavalry and would have relied on foreign troops to fill these positions in the army.

Where the Romans did benefit most was from their organization. This would allow legions to work cooperatively and attain higher levels of proficiency over time. Strategic resrves would be used to provide staying power which could whittle down the enemy's energy. If Rome could soak up an initial charge and draw out the fight then they could often find an advantage over less organized oppoents. So it kind of depends which Roman force you are throwing at the Gauls.

If the Roman army was fairly green then even with the tactical doctrine and better equipment at its disposal the army could still easily lose its morale. Remember that the heaviest losses are incurred on the retreat. Something like only 1-5% losses were expected in the main part of a battle. Once one side cracked they would be far easier to kill and might lose in excess of 20-30%. Especially prolific losses of 75% might occur if the retreat of an army was blocked by natural features or the enemy.

As for the Gladius. We are not really sure when it was incorporated into the army. It seems to have been of Spanish design and was a stabbing sword. This worked well with the Roman doctrine of staying in formation with your men and striking out from behind your shield when the opportunity arose. It's relatively smaller size was excellent for this task. Later the Gladius would fall out of favor in the late empire when Roman fighting styles began to diverge from this tradition.

I'd be happy to try and provide more specific answers if you have more questions.

Some accessible sources on this topic are "Roman Warfare" and "The Complete Roman Army" by Adrian Goldsworthy as well as "The Uniforms of the Roman World" by Kevin F. Kiley which contains good illustrations and brief descriptions of both Roman forces and it's opponents' over the years.

Furthermore if this type of thing interests you I have a YouTube channel where I use the total war video game series to make videos on Roman warfare. Not sure what the rules on self promotion are but I find that videos are often more effective than text. Channel is THFEproductions.

I am currently working on a 5 part series tracking the development of the Roman army over time. Right now though you could check out part 1 of my series on the First Punic War which covers Roman forces of that time or my 5 part series on the Battle of Cannae which shows a polybian army in action against Carthaginians (With Gallic forces)

Tl:dr, Depends on the type of Roman army you are dealing with. Mostly though the Gauls benefited from a society based around constant warfare which promoted the development of a strong warrior society. They also had elite units with good equipment and could draw on superior cavalry forces when compared to the Romans.

The Gladius was conducive to the Roman way of fighting which involved close order combat. The sword's small size was best used for opportunistic strikes from behind the large roman shield aimed at the opponents abdomen.