Where Gallic, Germanic, and Iberian tribes actually hunter/gatherers? By that time I thought Europe and the Mediterranean were dominated by farming cultures.
See e.g. First Farmers of Europe, https://www.amazon.com/First-Farmers-Europe-Evolutionary-Per...
I don't think small tribes of wandering hunters with small farming plots can sustain that many people. Caesar ran around and laid sieges to these things regularly during the Gaullic Wars. https://en.wikipedia.org/wiki/Oppidum
The Art of Not Being Governed
https://en.wikipedia.org/wiki/The_Art_of_Not_Being_Governed
which discusses in some detail the advantages of living in "marginally" farmable hillcountry.