If you trust the cover text, Spinrad tells the story of the events of Caesar' years in Gaul, mostly from the perspective of the young Avernian leader Vercingetorix.
His two main characters are mostly okay, if you don't look too closely.
Caesar advocated against capital punishment in the case of the Catilinarians and was never accused of murder, not even by his bitterest opponents. Together with Pompey and Crassus he formed a triumvirate, they did not share a consulship. His office of Pontifex maximus was for life. His perfect memory, attested by his ability to call out the centurions' names in the midst of a battle would not let him forget Marah. If he hadn't adopted Caesarion, he would have done even less for a foreigner.
Vercingetorix is not the innocent boy portrayed here, or a French version of Siegfried, even with the inclusion of a Brunnhilda - character (Rhia). For the slightest offence he cut off ears and pierced eyes of his soldiers. He made the war much bloodier and did not change the outcome: thousands died in vain for him. It is his responsibility to feed the Mandubians at Alesia, not Caesar's. (C., by the way, didn't either - barely enough food for HIS men.) However, it is quite possible that he was taken with Caesar's charisma, at first.
Spinrad is best when it comes to mystical scenes, the visions of Vercingetorix, written in the present tense, are the most readable part of the novel. I don't know enough about druidism to know if that was common practice.
However, as far as the battle scenes are concerned, they are PURE FANTASY. Spinrad really should know his stuff better, the Gallic Wars are easily available.
To name some of the grossest mistakes:
The Teutons were eliminated by Marius in 102 BC already. Caesar fought 120,000 Suebii in a tough battle, which was saved by P. Crassus' intervention. It was far from the easy skirmish Spinrad portrays.
Therefore his German cavalry are most probably Ubians who stayed faithfully with him all through 52 and the following years.
Avaricum had joined the Gallic cause long before Caesar's arrival. He took the town after a 25 day siege. Greek fire was never employed: Caesar wanted to get the food. Vercingetorix was close by almost the entire time but never showed the courage to fight Caesar directly.
And Gergovia and Alesia...
Well, read the commentaries on the Gallic War yourself!