The moral is about priorities. Just like in the story, if you put the big rocks (important things) in first, the small rocks (less important things) can still fit around them. It tells us to focus on the important tasks or aspects in life first, and then the minor ones can be dealt with.
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.
No, 'Rocks' isn't based on a true story. It's a work of fiction that aims to entertain and engage the audience through its unique plot and characters, rather than being tied to real-life events.