Celts descended from Spanish fishermen, study confirms

[Praefectusclassis]

Caesar discusses the Belgae at the beginnings of books 1 and 2 of the BG. He does seem to imply there that the Belgae are different from the other Gauls, but at the same time implies that they are not German either. The latter is made most clear (after a quick search) in 2.3: "that all the rest of the Belgae were in arms; and that the Germans, who dwell on this side of the Rhine, had joined themselves to them."
This sentence implies that a) Germans are not just those who live on the other side of the Rhine (and therefore the criteria are something else, prolly language) and b) Germans and Belgae are two different things.