Wikipedia says:
As many as a million people (probably 1 in 5 of the Gauls) died, another million were enslaved, 300 clans were subjugated and 800 cities were destroyed during the Gallic Wars. During Julius Caesar's campaign against the Helvetii (present-day Switzerland) approximately 60% of that nation was destroyed, and another 20% was taken into slavery.
Of course none of that is cited.
I recall reading that a cotemporary Roman source claimed Caesar's forces killed 3 million Gauls, but that was likely a significant exaggeration.
What is the truth?
Our primary source is Caesar himself. There are a lot of questions about where the line lies between truth and propaganda --For instance, his Commentaries massacres the Nervii as a fighting force almost to a man in their first major engagement, but they contribute mention-worthy forces to most of the coalition armies for the duration of his time in Gaul-- but you cannot argue with results: At the end of his war, Gaul was so beaten down that they didn't even rise up again in the decades following his assassination when the Republic and Empire was distracted, and before crossing the Rubicon he became the wealthiest man in the world off his share of captured booty and selling Gauls into slavery.