I've read many times the saying that Caesar's Commentaries on the Gallic War should be taken with a grain of salt, since they were written by Caesar as a propaganda piece for his own political purposes, rather than being a fair and neutral historical account.
While I don't doubt that Caesar, as an astute and cunning political leader, would be motivated to write in his own favor for his own political purposes, having actually read the book, it doesn't really read much like "propaganda" in the modern sense we understand. The book does not employ many black-and-white tactics we're accustomed to when thinking about "propaganda". It does not extensively preach morality, exclusively demonize the enemies, or sugarcoat Caesar's own actions. For example:
Caesar gives a fair and nuanced account of his enemies' perspective and motivations, such as as explaining the reasons of Helvetii migrations due to need, rather than a blind desire for conquest.
Caesar doesn't (exclusively) present himself as a liberator; rather, he explains his actions in rather realpolitique terms, such as preventing the Helvetii from crossing due to political reasons, rather than because it's "the morally right things to do".
Caesar acknowledges the patriotic motivations of leaders such as Vercingetorix, clearly describing the latter as a brave freedom fighter trying to save his people from Roman yoke. He doesn't paint Vercingetorix as some evil insurgent, nor contest his own actions as being that of a conqueror.
Caesar acknowledges the mistakes and errors of his own army, such as when one of his legions was rused out of their camp, ambushed and destroyed, or when he (almost) ran out of supplies due to putting too much trust in an unreliable ally.
Caesar makes no attempt to hide atrocities committed by his troops, sometimes under his command, such as ordering the slaughter of civilians in captured towns that refused to surrender. Granted, this may have been the normal practice of the day, but Caesar doesn't try to paint himself as uniquely merciful or benevolent, at least on those occasions where he is resisted.
Overall, if anything, Caesar's account of himself in the book reads like he portrays himself as a ruthless, efficient, and harsh but reasonable, leader. Comply with his demands, and you'll do well and be given mercy. Oppose him, and be destroyed with no mercy or remorse. And, having read the rest of Caesar's biography, it seems that he portrayed himself exactly as the kind of person he actually was, both at home and abroad.
Caesar's Commentarii obviously portray Caesar in the way he wanted to be portrayed, and in a way which would be well-received by a contemporary audience, who wanted a "firm and effective ruler" figure rather than an "angel of mercy" one. From this perspective, one could call the book propaganda. But I don't see it as "propaganda" in the sense of deceit, embellishment, morality preaching, or black-and-white demonizing, the techniques we commonly think of today when considering modern propaganda.
So, were the Commentarii really a "propaganda" piece, in the way we currently understand the term? Could Caesar have produced a politically favorable writing, yet one which preserved the truth, accuracy, and objectivity one expects of a reputable historian?
Outside of the internet and high school (maybe some intermediate undergraduate Latin reading courses) nobody refers to the BG or BC as "propagandistic." Caesar is understood by both historians and classicists to be writing a literary piece of some kind and employing rhetoric, even if he writes using the so-called Plain Style. In this he does not differ significantly from any of the other writers classicists read--if there was some sort of rule that we need to mistrust Caesar but automatically trust Cicero we'd be completely and utterly doomed. A major part--arguably the major part--of what classicists and ancient historians do when reading prose is working out the rhetorical ploys of the author. Caesar is no exception.
Why is Caesar spoken of on the internet as "propagandistic" and, for example, Cicero's accusation in the Second Philippic that Antony vomited on the Rostra while delivering a public contio for some reason gets a pass? I can come up with a few reasons. We do not know how the BG and especially how the BC came to be. H.J. Edwards, the editor of the Loeb, suggests in his introduction that the BG was an public edition of the dispatches (epistulae, or what Cicero often calls litterae) that Caesar, as most provincial magistrates and promagistrates, sent back to the senate. It's a plausible suggestion, but Edwards must admit that his reasons for thinking this are not grounded in especially rigorous scholarship: "in reading and translating them continuously, I have found myself unable to resist the impression..." Cicero thought that the BG and BC (the publication of the latter is much more complicated than the BG) were composed to act as notes for a history writer, and claimed that no one had used them as such because there wasn't really anything else to be said--although commentarii ("memoranda," so to speak) were common in the Late Republic and took very different forms, Caesar seems to be relatively unique. The version you'll find on the internet is Goldsworthy's, presumably because Goldsworthy is a pop historian (and thus accessible) with some credentials, even though he is not a classicist. Goldsworthy thinks the BG (and apparently the BC?) were composed on the road and read aloud at public contiones rather than in the senate. I find Goldsworthy's suggestion rather unconvincing, not least because if he's right the publication of the BG and BC would be apparently totally unique for commentarii as far as we know, but it's there. We really have no idea. The BC is even more troubling, partly because we really aren't even sure when it was likely published and with the civil war in full swing it's hard to tell who it was supposed to convince? Caesar's own supporters? Seems like a waste of time. Caesar's opponents? An equal waste of time. What was the rhetorical purpose? That last is the essential question: like every other Latin author Caesar wrote to convince people using rhetoric, but what was he trying to convince them of, and who was his intended audience? The details of publication might help us, but we don't have them.
But the real culprit is almost certainly high school Latin. No scholar finds the rhetorical punch of the BG (the BC is more obviously polemic) particularly noteworthy. In fact traditionally and for a very long time Caesar's rhetoric has been largely ignored by scholars as being simple and not particularly impressive, although this has been changing, and was not true in antiquity, in which Caesar was considered one of the masters of prose. So if anything the balance over time has been quite the opposite, that Caesar's polemic rhetoric is not especially impressive or noteworthy. It is significant that it is always the BG that is pointed out on the internet as being "propagandistic." The BC, despite being much more polemic, is consistently ignored--I admit I have no statistics on this, but I cannot come up with a single instance in my memory in which I saw the BC singled out on the internet, and a quick talk with some colleagues has backed this up. This fact is telling. The BG is traditionally the first unedited prose presented to intermediate Latin students, because its fairly predictable style and action-packed narrative are thought to make it both a straightforward and interesting read. Also traditional in the way we teach Caesar at the high school and intermediate undergraduate level is a talk explaining rhetoric in prose, which students have not yet encountered. Students are encouraged to consider carefully whether the precise letter of what Caesar is saying is true, and why he might be saying it if it is not. Good. But, either students do not advance into advanced Latin or do not realize that Caesar is not really being singled out--he is an easy example from which to learn the skill, not polemic rhetoric incarnate. Students all too frequently get to Cicero and don't seem to understand that Cicero is every bit as rhetorically polemic as Caesar and then some. Or they get to an undergraduate reading course and are stunned when their instructor asks them to start identifying and naming rhetorical devices.
So why is Caesar considered propaganda? He's not, at least not by scholars and not any more so than literally any other prose writer in Latin, as far as we can tell. But the exposure to Caesar (and, conversely, possibly the under-exposure to other prose authors among some audiences) has resulted in a very skewed understanding of Caesar at lower levels.