It starts at a specific point in the manga, but I can't recall exactly without looking it up. You might want to check some manga forums or dedicated fan sites for the precise chapter.
The following are some recommendations for novels where the main character is stronger than the system:
1. The Self-Cultivation of the Male Lead. Author: Wen Ruo Bucheng
2. I'm the Common Villain in the Fictions, by Big Firepower and Small Gun
3. The Number One Strong Control in History. Author: Old Xu
4. Low-Dimensional Revolution, by Fat...
Please note that these novels may only partially meet your needs. You are recommended to search further for more relevant novel recommendations.
It depends on how the author of the fanfiction has developed the plot and the characters. They could have given Natsu some special power - up or a unique situation that makes him stronger than Gildarts. For example, the author might have introduced a new form for Natsu that gives him an edge in combat, or perhaps they've weakened Gildarts in some way for the sake of the story. However, in the original series, Gildarts is extremely powerful, so this is a significant departure in the fanfiction.
In the 'izuku stronger than all might fanfiction', Izuku can be stronger for several reasons. Fanfiction writers often take creative liberties. They may write that Izuku has a deeper understanding of his quirk, One For All, compared to All Might. He could also have additional abilities or support from other characters that contribute to his increased strength. Maybe he has formed alliances with powerful individuals who help him train and unlock new levels of power, enabling him to surpass All Might.
Sukuna does die in the manga. The way it happens is quite dramatic and ties in with many other elements of the story, adding depth and tension to the overall narrative.