Well, without actually reading the 'ace whitebeards garp hits you fanfiction', it's hard to say for sure. But generally in One Piece fanfictions, it could be about battles, relationships, or character development. Maybe it shows Ace and Whitebeard facing off against Garp in an epic showdown, or perhaps it delves into their backstories and how they interact in a new, fictional situation.
You could try searching on popular fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. Just type in the title or relevant keywords in their search bars.
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'.
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.
Character development is key. Fanfictions explore how Ace and Luffy grow under Garp's influence. Maybe Luffy becomes more disciplined in some stories, or Ace learns to be more open - hearted, all thanks to their interactions with Garp and each other.