The Romans typically referred to them as Gauls, and the Roman name for the area of northern Italy they held was "Gallia Cisalpina" (or "Cisalpine Gaul" in modern English. The part of Gaul on "this side" of the Alps).
Ancient historical sources did sometimes use terms like "Celtae" to refer to some of these people, but who exactly they did and didn't use that for is very inconsistent. In modern parlance, Celt is usually taken to refer to the overriding language family (and its putative associated culture). One example of where these don't overlap is that the people we call "Insular Celts" were not considered Gauls (or Celts) by the Romans.
So if it was me, I'd use "Gauls" to refer to these people from the Roman point of view, and Celts when talking from my own modern perspective.