Caesar isn't really a scholar, but he claims in 'De bello Gallico' that the name 'Celt' was how one part of the Gauls called themselves. The Romans called them 'Galli':

tertium qui ipsorum lingua Celti, nostra Galli appelantur

and the third (tribe) are those that call themselves Celts but we call them Gauls

However, the Romans may have extended this name to groups of people who would not have called themselves by this name.

Wolfgang