This entire arc starts from the chapter 271 to be exact.

In my opinion, the most annoying thing the Author did was to never fix their character. He tries to put spotlight on multiple side characters like Claude, Alfoi and others but in my opinion, their constant unrestrained personality is more annoying than anything.

Here's a few details from the novel for those who are interested.
1) Ghislain had never encountered the ghost women in the mansion but instead he went to her instead.
2) Ghislain's original name for the product was 'Lovely Bling Bling' but everyone in the Fenris territory was against it so he reluctantly used the name coined by the Delphine house in his previous life-Deneb.
**MAJOR SPOILER ALERT**
3) The name Deneb actually belonged to the Saintess from a thousand years ago.

Nowhere really. He essentially becomes more and more of a thug who always causes trouble and constantly keeps on gambling. He becomes some of kind of leader to all the mages of Fenris territory since they are all basically slaves(except Vanessa). However, he does have his moments where he shine. One time, he gambled with a priest and won continuously so feeling wronged, that priest unconsciously called upon the Goddess to win the game but Alfoi still manages to win because the God couldn't stay for long. Other time, Ghislain and the Continent's other 7 strongest collectively fought against a dragon but their plans almost went up in smoke however, Alfoi somehow manages to prevent the Dragon from attacking even though he was only a 5th-circle while the Dragon was 9th-circle allowing Ghislain and others to win more 'easily'.

Right you are. In the novel, Gillian almost died with his chest pierced. Though it may differ in the manhwa since no mages or priest was present in the novel when they fought.

maybe you read it in demonicscans.