It was a Continental Celtic language with a fairly complicated inflecting morphology. It had six or seven cases. Unfortunately for the future of the language, it was, in fact, quite similar to Latin. This meant that the Gauls had little trouble learning Latin, and quickly adopted it. The Gaulish language survived up to the time of the Frankish conquest in some areas. Gregory of Tours mentions that there were still some people in his area who knew how to speak Gaulish at the time he wrote, in the sixth century. However, the language must have died out shortly thereafter.
Enough inscriptions and texts in the Gaulish language survive to enable us to ascertain that Gaulish was a P-Celtic language, having p where the other group of Celtic languages, Q-Celtic, have q or k. Thus the Gaulish word for "son" was *mabos or *mapos, where Q-Celtic would have had maccos or maqqos, forms which are actually attested in Ogham inscriptions. Some of the Celts in Iberia may have spoken Q-Celtic languages; their language has left even fewer traces.
The longest surviving extended discourse in Gaulish is the Coligny calendar, found in Coligny near Lyons, France, which recorded the months in use in Gaul at the time of its use, and marks each day as lucky or unlucky. Gaulish was written in various alphabets that were brought to Gaul by outsiders; the Greek alphabet was sometimes used, as were the Latin alphabet and the Etruscan alphabet. The ogham script, found in the British Isles, was not used in Gaul.
Some remnants of the language still survive in Breton, a British language, having borrowed some features from Gaulish, though this is merely a trace.