Thanks to both of you for your answers, very informative.

I just wanted to clarify, I do not think that gauls were so 'inferior' to legionaries, not at all. In fact, the celts are one of my favorite civilzation of the ancient period, I just really love their culture. The distinction I made was mostly the drilling of the troops. Learning to move as a unit. For example, a unit wants to let another unit pass through them, so they would form in columns, leaving space for the others to come through. Another example would be turning to face foes in the flanks, or changing direction by wheeling, changing the pace, faking retreats, and all those things that professional soldiers can do, because they're trained to. They know how to act as a unit.

Of course, gauls also acted as units in some ways, combat for example. But troops without drilling can not make such complex maneuvers, and although they may fight together in shieldwalls or supporting each other in a battle line, that's very different from the tactics one can develop with drilled troops.
Also, although maybe veteran warriors and excellent fighters, I seriously doubt that untrained troops could hold the line as trained troops did. Simply because when all seems lost, trained troops are probably going to hold the line for longer, because they know things untrained troops don't.

Yes, maybe some of them participated in the legions and phalanx formations, but did the gauls actually had drilled troops? Troops that could perform complex maneuvers and were actually trained together as a unit to fight? I know they fought in units, and Solduros for example were extremely brave, and held their line no matter what, but that's not the same as being drilled or trained.
Of course most people overestimate the romans, but the truth is they had a battle drill and they knew how to march, how to function properly as a unit on the battlefield, and how to accomplish complex tasks. I know that one on one, any gaul could beat any roman, and maybe a devastating first charge would break the roman lines and give the gauls a victory. But if that didn't work, they were really in some trouble.

So maybe at some point the gauls (and yes, I know they were not actually a nation, of course) realized it was more effective to actually train troops to increase their effectivness, and not just trying to beat the enemy with sheer shock tactics. Maybe they didn't, and the professionalism of the Neitos means they had excellent equipment and were extremely good fighters.

In my game, there are different stats and different phases. In the Impact phase for example (almost just like Field of Glory), Gauls do have the advantage, being extremely heavy shock troops. But after that, in the melee phase, if the legionaries do not lose their cohesion as units, they have the advantage, because of the close order and discipline (and the fact that gauls historically discouraged quickly if the enemy did not break quickly). And when things start to go badly for any of the units, what matters is morale. Of course, Solduros have incredibly high morale, so they won't run. But non professional or non noble troops have much less morale than actual trained and professional soldiers like Legionaries or (if they're finally included in the game) Neitos.
Also, in the movement phase, drilled units can move much better and perform more actions than undrilled troops, simply because they're trained to move as a whole, and they're not just a lot of good warriors walking or running together.

To add a few more questions, how high would the percentage of chainmail equipped troops be in a gallic army? I read somewhere, sometime ago, that probably less than 30% counting both infantry and cavalry, but maybe some rich chieftain could even get 40% of their troops equipped in that way. Also, as you say Brennus, the Neitos would be outnumbered by less professional troops and part-time soldiers, but by how much?

Another, not so related question, but still talking about gauls. From what I've read, Solduros and Gaesatae aren't contemporary. Gaesatae appeared probably earlier and dissapeared before the solduros even existed. But the thing is, the only mention of solduros is by Julius Caesar, so, maybe they had existed for a long time before he mentioned them in his De Bello Gallico.

Any other contributions to the composition of celtic armies (both early and late) would be enormously appreciated. I really enjoy reading about this, but I really can't find so much books on this.

Thanks for your time guys, I enjoyed reading you!