The cladogram above represents my understanding of the development and relationships of the Celtic and Celtoid languages. A few comments on this…
I have indicated four distinct Continental Celtic languages (Gaulish, Cisalpine Gaulish, Belgic and Galatian), but these may have been dialects of a single language rather than distinct languages. There is simply not enough evidence to know for sure.
You won’t find Lepontic in this cladogram because I don’t believe that Lepontic was a Celtic language. Although I have thought of it as an Osco-Umbrian language, I now prefer to think it might have been an Illyric language.
The grey branch labeled “Vindonian” represents a Continental Celtoid conlang that I have been developing. Vindonian would hypothetically have been spoken among the Celtoid peoples that lived in northwestern France before the westward expansions of Celtic from Central Europe, and would be the sister language of the Brittonic and Goidelic languages.