The book says 'text by Minekura Kazuya', and while I doubt she wrote all of it one may assume it had her approval. This makes it pretty much the decisive word on canon. So I went and squinted at the Goujun passages:
On the vexed question of Is Jiip Goujun's reincarnation? the main entries for Jiip and for Goujun say not a word about them being related to each other. The entry for Jiip gives the novel version of his creation as canon, which annoys me for various reasons, not least of which is that the novels are truly execrable. Things get a bit better in the Comparative Analysis section- which compares the Chinese version to the Minekuran redaction, as well as mentioning various divergences among the manga, anime and novels. I'm assuming she wrote this part, if only from the colloquial tone and the number of (grins) in the text.
Under Jiip, we have "the Saiyuuki novels divulged the settei that 'Jiip too was an itan being, produced from the mixing of magic and science' but if you think about it the manga hasn't yet (said one way or the other)..." The bracketted part is my interpretation of the text's laconic and typically Japanese 'the manga hasn't yet.'
In the Goujun section she touches on the Chinese story of how Gokuu gave Goujun a hard time and how it was the dragons who gave Gokuu his nyoibou in order to get rid of him, then continues: "In the manga there's a nuance that this Goujun is the previous incarnation of Jiip, but in the novel the 'dragon horse' Sanzou rides on is Goujun's son."
Nuance, you say. Mhh. Crumbs, yes, but I shall make do. The problematic part is the main section description of Goujun:
The person in top control of the army of the west, Tenpou and Kenren's commanding officer. Soldierly and a stickler for the rules, he considers the orders of Heaven to be absolute and unquestionable. Possesses an exact precise nature which dispenses with nonessentials. While holding Tenpou in high estimation, when Tenpou suddenly begins to associate with Gokuu and the others, Goujun grasps very early on that the four of them are a dangerous element. After getting involved in their conspiracy he can see where their paths will take them even though he is unable to understand their motivation.
Physically: two horns protrude from his head amidst silver hair. Porcelain-white skin with scales here and there; eyes red and gold. His unusual physiognomy is much closer to a youkai's than a kami's.
That 'regards Heaven's commands as absolute' depressed me for a bit: OK fine *my* Goujun is AU now-go-away. However what becomes clear from translating is that this section reads different from the analysis section and the Q&A sections. I think it was written by a staff writer in the good bland Animage tradition of stating only the obvious facts contained in the manga/ anime itself. This is how Goujun looks to an uncritical eye. Any suspicions about unreliable narrators or possible hidden depths won't get treated here. Now doubtless westerners and yaoistas have a bad habit of thinking too precisely on the character and interpreting in possibly fanciful ways. No matter. I see depths in Goujun (I can come up with five explanations as to exactly *why* he springs Kenren in the first place) and I shall go on assuming he has unstated doubts about the whole of Heaven.