It also seems that later writers subsequently used Caesar's work as a source:
'Britain is triangular in shape; and its longest side stretches parallel to Celtica, neither exceeding nor falling short of the length of Celtica; for each of the two lengths is about four thousand three hundred — or four hundred — stadia: the Celtic length that extends from the outlets of the Rhenus as far as those northern ends of the Pyrenees that are near Aquitania, as also the length that extends from Cantium (which is directly opposite the outlets of the Rhenus), the most easterly point of Britain, as far as that westerly end of the island which lies opposite the Aquitanian Pyrenees. This, of course, is the shortest distance from the Pyrenees to the Rhenus, since, as I have already said, the greatest distance is as much as five thousand stadia; yet it is reasonable to suppose that there is a convergence from the parallel position which the river and the mountains occupy with reference to each other,since at the ends where they approach the ocean there is a curve in both of them.' - Strabo Geography IV 5.1
Anyway, back to the topic: are there any other sources on the Gallic War that act as a counterbalance to Caesar's account?