It would be foolish to think the Gallic Wars happened precisely as Caesar wrote because his book was overt self-aggrandizing propaganda intended to boost his standing with the Roman public.
But as to the Gallic Wars happening at all? There is corroborating evidence. Other Romans wrote about it, often to damn it, and they treated the existence of the war as a fact. He brought tons of Gallic slaves back to Rome which would corroborate the most basic facts of the claim (e.g. that there had been wars with Gallic people) to other Romans who weren't there to personally witness it. Furthermore there is modern archeological evidence of battles and massacres which roughly line up with some of what Caesar claimed to have done.
Historians know that they can't take written sources at face value. For them, Caesar's account of the Gallic Wars is just another piece of evidence, not a Definite Proof of Everything That Was.
Caesar is not considered an infallible authority, but that has no bearing on the general consensus that Gallic Wars actually took place.
But as to the Gallic Wars happening at all? There is corroborating evidence. Other Romans wrote about it, often to damn it, and they treated the existence of the war as a fact. He brought tons of Gallic slaves back to Rome which would corroborate the most basic facts of the claim (e.g. that there had been wars with Gallic people) to other Romans who weren't there to personally witness it. Furthermore there is modern archeological evidence of battles and massacres which roughly line up with some of what Caesar claimed to have done.