Well, that could change the entire dynamic of the story. Maybe Shadow would have to deal with the guilt of Sonic's death and strive to fulfill Sonic's unfinished missions.
It would cause chaos. The balance of society would be disrupted as there would be a sudden increase in population and many logistical and resource-related problems.
It could lead to major changes in the story. Goku might struggle more to raise the child alone and his character development could take a different path.
If Gohan died in the Cell Games in fanfiction, it would be a game-changer. It could trigger new plotlines and character developments. Maybe Vegeta would have a change of heart or new allies would emerge to help the Z Fighters.
The story would likely take a very dark turn. Other characters might step up and try to fill Gohan's role, but it would be a huge challenge. It could also change the outcome of the Cell Games completely.
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'.