Any number given for the population of Gaul would have to be only a rough estimate, as there were no censuses or other accurate measurements taken on the population of the area as a whole. There are some estimates of the Roman population of Gaul at the death of Augustus in 14 CE, which would be just 60 to 70 years after the Roman conquest, so the numbers would be similar. Historian Karl Beloch (1886) estimated Roman Gaul to have a population of 4.9 million. Other estimates include 5.75 million (McEvedy & Jones, 1978), and 5.8 million (Frier, 2000). Based off of these estimates by historians, I would wager that the population of Pre-Roman Gaul was somewhere around 5 million people.

Source: Contours of the World Economy 1-2030 AD: Essays in Macro-Economic History, 2007, Angus Maddison

Follow up questions: how urbanized was it? and were there any large population centers? What was life like in a Gallic city/town?

Follow-up question: How many tribes composed the population of Gaul and what was the usual size of these tribes?