I don't have enough context to determine exactly who 'garp' is. It could be a newly created character by the fanfic writer, or it might be a character from a relatively unknown story that is being incorporated into the 'ace fanfic'.
One common theme is the clash of ideologies. Akainu has a very strict and uncompromising view of justice in his original story, while Naruto has a more forgiving and rehabilitative view. So when they interact in fanfiction, this clash often forms a central theme.
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.
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.
I'm not entirely sure specifically what 'garp save ace fanfiction' is about without more context. It could potentially be a fan - made story in which Garp tries to save Ace within the context of a particular fictional universe, perhaps related to One Piece.