He absolutely did commit (or allow) war crimes (AVARICUM). But not what we would define as 'genocide' (as per the UN or Lempkin definitions) - the intention to destroy a group of people in part or in whole.

For a start, there's the issue of numbers. It was the later historian Plutarch, not Caesar, who gave the 'killed 1 million, enslaved 1 million out of a population of 3 million' number. This number, as with all ancient writers, should be taken with a heavy grain of salt due to the immense size and is most likely an exaggeration. Modern archaeological evidence instead seems to suggest that the pre-conquest population was in the 8-10 million range instead.

Most of the Gallic population, it seems, survived the war considering the fact that later Roman villas have been mostly found on the site of old Gallic farms. This indicates a continuity of elites in Gaul which followed a Roman model. As we now understand it, Roman 'colonisation' was not colonisation in the same sense that we think of today with the European colonial empires of the early modern period. It was a conquest, not a colonisation. The native population was not inhibited from social mobility within the new social order and the land wasn't made poor for the benefit of the metropole. Gaul was integrated into the Roman state, not destroyed, and Celtic culture continued to exist as late as the 4th century AD.

So, on a national level, Caesar didn't commit genocide. His motives for conquest were based on gaining personal prestige and glory, not a desire to ethnically cleanse or destroy a block of peoples to then completely repopulate the land with his own. The fact that he was willing to work with other tribes during the conquest and had no problems ruling over the Gauls in the already Roman provinces of Cisalpine Gaul in northern Italy doesn't point to him having any issues with the Gauls on a racial basis.

But what about on a localised level, where individual tribes apparently were destroyed? This is something which, again, doesn't seem to fit the term genocide. Most of the major recorded casualties of the conquest occured after the various battles, when the retreating men were pursued and cut down by the Romans. This was fairly standard practice for armies in the day (and can be seen in other examples like the Second Punic War with Hannibal) and was about stopping the enemy from regrouping, not destroying their ethnic character.

And in numerous other examples, the families of the combatants in battles were pursued by the Romans not to exterminate, but to enslave (ancient Romans saw war prisoners and their relatives automatically as slaves). Horrible? Yes. Genocide? No, the intent was financial, not racial/ethnic. The Gallic war was filled with mass killings (as would be the better word to use in these circumstances) but not mass genocide.

It baffles me how, when addressing this topic, someone can go on something like r/askhistorians and find about three posts responding to Gallic genocide question where the response is a resounding, informative 'no'. But whenever discussion of the topic crops up in the Romaboo subreddits or other online spaces it's taken and discussed as an undisputed fact that 'yes, genocide was committed during the Gallic Wars'.

Edit: Seeing as there's been a bit of pushback here and there, I think it's worth linking posts from r/askhistorians responding to this topic:

Edit 2: Actually, the top comment mentioning the Eburones as an example of genocidal action during the Gallic wars is completely right. Even if it's believed that Caesar didn't completely wipe them out, there was still an intent to do so regardless of how much he embellished what happened. I will remove my comment responding to it and apologise for misunderstanding and looking over that example, especially when looking over the fourth link I posted the genocidal intent there isn't properly pushed back against. Sorry about that.