Alright, so this assumes Gaul is a single centralized state. Which it is not. It is a region inhabited by a lot of warring chiefdoms which banded together in confederations. Vercingetorix tried to unite them against Caesar, but they failed.
And it also assumes the Romans were not already active and in Gaul in the time of Caesar. Which they were. They ruled the richer southern parts of Gaul, which were called Narbonensis. One reason they went up north was to secure it from the raids of the aforementioned warring chiefdoms.
That's an alternate history, I mean Kerney mean WI the best case scenario for the Gauls.
If I remember correctly Italy was a place where everybody fought everybody for centuries and Italy did not show any slightest inclinations to get united, but a little town of Rome changed the situation.
If we look at the Gauls this place is much more homogenous and have in common than Italy before Rome.
And Julius Caesar before Alesia made a great effort to make the Gauls feel like they are one entity - these famous all-Gaullish gatherings of Caesar.
If Alesia is a Roman disastrous defeat, Caesar will be remembered for centuries like the guy who united the Gauls (against himself).
But this unity might stick, Vercingetorix would be renowned as the greatest man alive and given that he was quite young and not assasinated, he might make the foundations of the Gaullic unity.
It must not be necessarily the Celtic Empire like centralised unified state. It might be something like Medieval Holly Roman Empire. Not too much of unity but able to gather a big army in time of emergency.
Being between the Roman Empire and the ferocious Germanic tribes begs for some sort of coordinated military effort and unity.
So why not?
I mean, there is no such thing as "these tribes are not able to unite at all under any circumstances".
I don't have in mind the united Gaul conquering the Roman Southern Gaul and burning Rome to the ground. No, they stay in their part of Gaul and having in mind the Germanic threat, may be there is some place for coordinated effort of Rome and Gaul against some Germanic invasions.
p.s. I just remembered that good part of Spain had been united by the Barcids (from Carthage). So sometimes what you need is good circumstances and an able man in charge.
Dacian tribes had not been too united before, but at Julius Caesar time they surprised their neighbors by creating some sort of a united polity.