I'm not sure specifically as there could be many interpretations of such a fanfic. It might involve a scenario where Guts, a character, kills Griffith in a fictional story created by fans. Maybe it's about Guts seeking revenge on Griffith for some past wrongs in the context of the original story's lore.
Perhaps in this fanfic, Griffith has turned into something truly evil and a threat to the world Guts cares about. Guts, being the warrior he is, might feel it's his duty to put an end to Griffith to save others. Also, Guts could be driven by his own sense of justice. If Griffith has wronged innocent people, Guts may decide that killing him is the only way to set things right.
It depends on the specific fanfic. Some fanfics might have Guts killing Griffith as a major plot point, while others could take a different direction and not have that happen at all.
Finding fanfics can be a bit tricky. You could start by searching on popular fanfic websites like Archive of Our Own (AO3) or FanFiction.net. Just type in 'guts kills griffith' in the search bar and see if it comes up.
Most likely not. The entertainment industry is full of unfounded rumors. Just because a statement exists doesn't mean it's true. Childish Gambino is known for his talent in music and other creative works, and it's unfair to assume there is such a story without proper proof.
Guts is purely fictional. It's a concept or character that exists within the realm of imagination and creative storytelling rather than being based on reality.