The number of copies is a test for accuracy of transmission - not for the accuracy of the original text.
Caesar's Gallic Wars was written by a participant in those wars, during the campaign.
The Gospels were written decades after the event by unknown people. None can be definitely identified as being present at the events they describe (for some events we know that they were not). We can be sure that at most one - more likely none - of the authors of the Synoptic Gospels was present.
Given those facts there is good reason to trust the Gallic Wars over the Gospels.