Populations estimates in ancient times tend to be difficult at best to assert in the absence of hard data : we know that censuses were taken in independent Gaul by its various petty-states (probably for fiscal and mobilization purposes) and we know when Roman administrations enacted them after the conquest. However, we do not have the results : thus, populations estimates you can find around are largely based on both broad fragmentary estimates by ancient authors (i.e. population of a given agglomeration or army) and archeological data.

Currently, an average of 10 millions inhabitants in late independent Gaul (IInd-Ist centuries BCE, including southern parts) is more or less agreed on, give or take two millions : this is mostly based on renewed understanding of the region as an agricultural powerhouse whose anthropisation (meaning the human transformation of landscape) was much more important than previously thought. You might find more elements of answer on the diversity of Gaulish agriculture in this earlier answer, but the main notion is that it was perfectly able to feed such a large population and then some more, thanks to a widespread iron tooling (scythes and sickles might have been significantly widespread in the western provinces from Gaul; furrow ploughs), fertile lands, ten of thousands farmsteads, an important fluvial and road network, etc.

This agricultural surplus was probably a main cause in the making of Halstattian and LaTenian agglomerations in the Vth/IVth centuries and IIIrd/IInd centuries BCE, which was not limited to Gaul but included a good part of southern Germania as well, which essentially partook to the same civilization until the conquest of Gaul. While due caution should be applied into describing the political make-up Romans were confronted to, especially because (as in ancient Greece) what was true of a people was not necessarily so for another, we shouldn't consider these populations as "tribes" (a problematic notion of its own, in that it implies social simpleness, whereas it could be more easily understood as an inbricked identity as much present in Rome than in Greece non-exclusive to social or political sophistication), but as rather developed societies and in this case, with demographics comparable to late Republican Italy, a region with which Gaul was deeply connected to already before the conquest notably trough wine trade from the peninsula, less so by exportation of transalpine products.

It doesn't mean the Roman conquest did not change things, far from it. But the romanization of Gauls was a complex process that, besides an voluntarist imperial policy, largely involved indigenous reality transformed on Roman lines especially trough regional elites. These seem to not only have remained largely in place after the conquest, in so far they accepted or even welcomed Roman takeover (whereas anti-Roman factions and leaders seem to have been violently crushed) : besides the important urban transformations (involving the more or less gradual abandonment of most oppida in favour of transformed plain Roman cities either transformed from earlier agglomerations or newly built) Gallic elites served as a strong cultural and social transmission belt in the countryside. For instance, as Roman roads in Gaul were largely traced upon older indigenous ways, Roman villae there tended to be built on or nearby old farmsteads along with a new land registration (together with the introduction and systematisation of Latin and Roman Law in the provinces).

The make-up of important demesnes in Northern Gaul and the pursuit of a Roman-style rationalization of agriculture also brought some practical changes as the vallus, a non-mechanical reaper device but it did probably not led to a greater production, rather being considered "economical" in relevance to latitudinal production. Roman influence on agriculture wasn't limited to its legal and social romanisation : several crops were introduced in Gaul (or, as it's becoming increasingly apparent especially with wine or oil, less introduced than booming) such as asparagus, artichoke, cabbage, cucumber, leeks, lettuce, garlic, onions, etc. although its influence on local diet is not clear, it was part of a commercial change that saw Gaul becoming an exporter of high-value crops as wine.

Thus, it's not obvious that the demographic recovery after the Gallic Wars, which losses (due to both warfare and a severe drought) might have amounted to a tenth of the population and particularly striking northernmost regions, was due to Roman presence : overall, the population of Roman Gaul seems to have been fairly close to what it was before, around 10 to 12 millions at its apogee perhaps, whereas the previously comparable situation in southern Germania significantly changed, with its "agricultural system in disarray" (Peter S. Wells) as a consequence of both Germanic pressure but probably at least as well of the Roman conquest of Gaul and the severance of ties with Gaulish petty-states.