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'.
Growth is a very common theme. Since Coby starts as a relatively weak character, many fanfictions focus on his journey to become stronger, both physically and mentally.
One possible theme could be friendship. Coby Morgan might form strong bonds with other characters at the academy. Another theme could be self - discovery. As he goes through his experiences at the academy, he may learn more about himself.
There is a fanfiction titled 'Admiral Coby and the New Era'. It's really interesting as it shows Coby in a future where he has to deal with new threats to the world. It delves into his strategic thinking and how he uses his influence to keep the peace. The author does a great job of portraying Coby's character development.
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.