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.
I'm not sure specifically as it's a fanfiction. It could potentially involve Maleficent using her dark powers to intimidate Mal who is pregnant, perhaps to gain some sort of advantage or to fulfill some dark plan. But without reading the actual fanfiction, it's hard to be certain.
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.