We're going back and forth quite a bit. It's not that productive.
First, it is a mistake to take the statistics, or, for that matter, many statements of ancient historians as gospel. A well-known example is the Persian force at the Battle of Thermopylae. It wasn't a million. It was large, but nowhere near one million. There is a lot of legend in ancient sources.
The entire population of Gaul at the time of Caesar was ~1 million. The notion that he took 400,000 slaves must be greeted with some skepticism.
I want people to understand that ancient mass movements of people were not possible like they are in the Jet Age, or even as frequent as they were during the Germanic upheavals at the dawn of the "Dark Ages." A rather standard ancient warship, the quadrireme, carried about 75 people, 100 if packed.
(This is why I laugh at notions of the Anatolian--->Etruscan mass migration proffered by Herodotus, as more legend than fact. A "starving" inland nation of, say, 200,000 travels to coastal Anatolia, in the hostile territory of their neighbors, and cuts down enough trees, and builds 2000 ships?)
Always keep in mind the logistics. If they sound hard to believe, they are.
Caesar took 400,000 Gaulish slaves? How were they transported to Rome? Where are the mass graves from the tens of thousands who must have died en route?
Surely some went to places other than Italy. Why is there no Gallic genetic signature in North Africa? It had been solidly Roman by the time of Caesar, for about ~200 years (the age of our country).
Think it through. Healthy skepticism with logic are both your friends.