Can they be verified as from his own time by ancient testimony?
Here is what good old wiki* has to say:
The original publication time of the Bello Gallico is uncertain. It had been definitely published by 46 BC, when Cicero reviewed it and gave it great praise.
And the footnote 15 directs us to this** passage:
But Caesar, who was guided by the principles of art, has corrected the imperfections of a vicious custom, by adopting the rules and improvements of a good one, as he found them occasionally displayed in the course of polite conversation. Accordingly, to the purest elegance of expression, (which is equally necessary to every well-bred citizen, as to an orator) he has added all the various ornaments of eloquence; so that he seems to exhibit the finest painting in the most advantageous point of view. As he has such extraordinary merit even in the common run of his language, I must confess that there is no person I know of, to whom he should yield the preference. Besides, his manner of speaking, both as to his voice and gesture, is splendid and noble, without the least appearance of artifice or affectation: and there is a dignity in his very presence, which bespeaks a great and elevated mind."
 "Indeed," said Brutus, "his orations please me highly; for I have had the satisfaction to read several of them. He has likewise written some commentaries, or short memoirs, of his own transactions;"
"... and such," said I, "as merit the highest approbation: for they are plain, correct, and graceful, and divested of all the ornaments of language, so as to appear (if I may be allowed the expression) in a kind of undress. But while he pretended only to furnish the loose materials, for such as might be inclined to compose a regular history, he may, perhaps, have gratified the vanity of a few literary embroiderers; but he has certainly prevented all sensible men from attempting any improvement on his plan. For in history, nothing is more pleasing than a correct and elegant brevity of expression. With your leave, however, it is high time to return to those orators who have quitted the stage of life.
First, this is not much more specific as to what the Commentaries were about than St Papias is specific about the content of the Gospels. Not having Papias at hand right now .... I look it up. We don't have him in the original, but referred to*** in St. Irenaeus and in Eusebius.
Irenaeus stated that Papias had heard the apostle John preach and also knew Polycarp. Eusebius mentioned his Explanation of the Sayings of the Lord. In the preface to this work Papias maintains that his primary purpose was to bring forth a truthful record of a collection of the words and deeds of the apostles that were told to him by a presbyter. Irenaeus understood him to be alluding to the apostle John, but Eusebius contended that he referred to two Johns, one who was the apostle and the other who was the companion of Aristion.°
Papias claimed that Mark, the Evangelist, who had never heard Christ, was the interpreter of Peter, and that he carefully gave an account of everything he remembered from the preaching of Peter. Papias affirmed the statement that Matthew wrote down sayings of Jesus in Hebrew. Irenaeus understood this as a reference to Hebraisms in Matthew’s Gospel, whereas Origen took this to mean that Matthew originally wrote his Gospel in Hebrew.
The little we have from Papias has been pretended to give us too little to identify the Gospels, but the passage in Brutus by Cicero doesn't actually tell us whether the Commentarii of his own transactions were from Bellum Gallicum, from Bellum Civile or from his fortunes as a soothsayer as Rex sacrificulus, when he invented the Julian calendar or about sth now completely unknown. They are equally bland.
UN NOUVEAU MANUSCRIT DU " BRUTUS " DE CICÉRON
Le Monde : Publié le 21 octobre 1957 à 00h00 Mis à jour le 21 octobre 1957 à 00h00
There is a little hitch.
Article réservé aux abonnés
I'm not a subscriber to LeMonde.
Douglas Ross Thomas, Trinity College, University of Oxford may have provided a good way of obviating the lack, since A thesis submitted for the degree of Doctor of Philosophy Trinity 2021 is called THE TEXT AND TRANSMISSION OF CICERO’S BRUTUS.°°
I begin Part One with a catalogue of the c.109 extant manuscripts of Brut. The second chapter of this part is devoted to the oldest surviving manuscript, the Cremona fragment. I demonstrate, against the view of earlier scholars, that the Cremona fragment is a part of the long-lost Codex Laudensis (L), the archetype of the entire tradition.
It seems the Cremona fragment is from the IXth C. AD, like the Caesar manuscripts.
In overall 107 extant manuscripts, the others are from XVth C. After a rapid scroll, I don't recall any that's older than 1421. Some are imprecisely dated "s. XV." or "1425 - 1500" and some have very precise dates, like
100. Venice, Biblioteca Nazionale Marciana, Lat. Z. 420 (1509) = Cy
Parchment; Parma; 11 February 1428.
"11 February 1428," that's as precise as a blog post! Some are in between, like
96. Vatican City, BAV, Vat. lat. 3238 = Cu
Parchment; Florence; 1423-c.1429?
It's not 75 years or 100 years, it's also not to the day, but it's within a decade.
The point is, all except the IX C Cremona fragment are centuries younger, more specifically the XVth C. than the oldest manuscripts of Caesar's either Gallic War or whole corpus.
A historian doubting the authorship of Matthew, Mark, and (within two possible candidates) John would logically also have to doubt Caesar's authorship to works he's said to have written.
But who said Freethinkers are above cherrypicking? Not me!
Hans Georg Lundahl
St. Gregory the Thaumaturge
Neocaesareae, in Ponto, natalis sancti Gregorii, Episcopi et Confessoris, doctrina et sanctitate illustris, qui propter signa atque miracula, quae cum multa Ecclesiarum gloria perpetravit, Thaumaturgus est appellatus.
*** Summary of both mentions from:
° According to the priest Jean Colson, precisely the presbyter was the beloved disciple - while all the Apostles fled (including the Sons of Zebedee), the beloved disciple was around next day at the Cross. If he was a Cohen, as Colson thinks, it explains also how he could the same day have the Blessed Virgin as adoptive mother in his home, without leaving Jerusalem.