I know his own "diary" was a major source of our knowledge about the campaign against the Gauls. but is Caesar's writings the only source of our knowledge of the war really?
Caesar's commentaries on his Gallic adventure were not for his own private use (as in a diary), but for political propaganda. It's useful to note in this context that Caesar refers to himself in the third person ("Caesar", "he") and not in the first person ("I"). Unless Caesar were quite mad, it's unlikely he'd use the third person for himself in his own diaries. (Though this is someone who claimed to be a descendent of Venus...)
We do have other sources for the war, but I don't think they're really dispositive of the numbers Caesar faced. Our other sources are archaeological -- so, we can go to Gergovia (say) and excavate for Roman war machinery, but it won't tell us how many Gallic women and children were clapped in chains and sent to Rome. For that kind of information, we rely on Caesar (who claims to have enslaved a million men, women and children).
You said the commentaires was for propaganda purpose, and so think I, but the talk abput descent from venus is too of course, i dont think he really thought he was a desent from a god, but it strengthen his stand with the people.