Well..... celts don’t really exist, it’s a modern myth. There was no Celtic nation or identity, it was a handy term created to lump the various tribes that developed LeTenu style. It’s like the generic term applied to the First Nations...native Americans. They aren’t Americans as the Americans are the incomers. The various tribes aren’t one nation and they all refer to each other by their tribe so a Dakota Sioux will refer to a Cree as a Cree not a Native American.
But, in wargame terms, a Celtic army would cover Gaul, Britain, Teutonic tribes, and Hibernians. As they are all Celtic tribes, and pretty much fought the same. Gallic forces would be good as they can double up as any of the others to fight the Romans or each other, which they did a lot.
Gauls will be good as you can have asterix et al leading them in a fantasy version.
Nige