Celtic peoples were called different names: Gauls in France, Belgae in Northern France, Galates in the Balkans and numerous tribal names everywhere. But there is no doubt that they all spoke one language, or similar varieties of the same one. This comes from town names, inscriptions and Celtic words written down by Greek and Roman authors. Their language system is what is called "Classical Celtic": it was very close to the Italic group of tongues, and Julius Caesar even had to write his letters to his legates in Greek for Gaulish leaders not to be able to read them if they might happen to gain hold of these missives. He did so because Latin could be understood by Celts quite well without having had to study it.
Gaulish was highly inflected, but had practically nothing in common with Insular Celtic morphology and phonetics: it had no initial mutations, had an ordinary Indo-European word order (subject - predicate - adverbial modifiers) and grammatical forms similar to those of the Proto-Indo-European model. Gaulish is referred to as being in the so-called "P-Celtic" group, as it uses p in place if the indoeuropean kw.
As Gaulish was quite similar to Latin it was quickly assimilated as
early as* the last centuries B.C. within this very large language area.
Popular Latin borrowed many words from Gaulish,
and even now in French, Italian and Spanish there is a fair number of Celtic
words. But even yet not too much is known about Gaulish, its structure