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.
It's very unlikely. Publishers need to make money from their works, and offering the whole book for free online would undermine that. However, there might be some limited - time offers or promotional events where you could get access to read it for free, but these are rare.
There are some websites that might offer free graphic novels, but many of them may be operating illegally. It's best to check if your local library has an e - library service where you might be able to borrow the 'Guts' graphic novel digitally for free and read it in full legally.
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.