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'.
In Star Wars fanfiction, 'Roger Roger' can be used to set the mood. If a story is set in a droid - heavy environment, using this phrase helps to build the atmosphere. For example, in a story about a droid repair shop, the droids might say 'Roger Roger' when receiving instructions on repairs. It also helps to distinguish the droids from other characters in the story, as it is a very specific form of communication associated with them.
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.
One popular garp fanfiction story could be about Garp's early days in the Marines, his training and the friendships he formed. Another might focus on his relationship with his family members in a different light, like how he interacted with his grandson Luffy during Luffy's childhood. There are also stories that might explore Garp's role in some of the major events in the One Piece world from a different perspective.