The book you want is Williams' Beyond the Rubicon (2001). Here is a BMCR review. It is a study of both the history of Cisalpine Gaul and the historiography of the same from the 4th century BCE to the point (probably in 42 BCE) when it became incorporated into Italy as part of the ratification of Caesar's acts after his assassination.

Williams argues that the Celtic invasion of 225 BCE and the subsequent pacification (well, for a year or so) of the Po valley was perhaps the key moment for the imagination of Italy as a political community. The Italian states recognized they had a common interest in repelling the Celtic "invaders" (though I think if you looked at Celtic historians you would find a different story), and that idea of communal interests eventually developed into the unified Italy of the Julio-Claudians.

That thesis doesn't, in my memory at least, account (and if anyone of you have a problem with the split infinitive, fite me) for the defections of the southern cities during Punic 2, or for some explanations of the defections during the Social War, but at least the conception of Cisalpine Gaul as a part of Italy Williams established in the histories very early.

Already in Cato the Elder (d. 149 BCE), the first historian to write in Latin, and of whose work we have a bit left, we get phrases about the Alps being the shield of Italy and other such things. So the close association of Cisalpine Gaul with the geographical region Italy popped up basically as soon as Latin history did, even if CG (can't keep typing all that out) wasn't administratively incorporated into Italy for another hundred years.

Since every surviving Roman historian writing in Greek or Latin postdates that 225 invasion, they all reflect the understanding of the Italian political community that arose from that invasion.