It could be an old and powerful wizard who has been in hiding for years. Maybe he was a friend of Dumbledore's but had some differences with him, so he stayed away from the main events in Harry's life until now.
To start a 'Harry Raised Grandfather' fanfic, begin with character development. Figure out what kind of person Harry's grandfather was originally and how Harry's presence will change him. Then, create a plot outline. Maybe start with Harry arriving in the past in a dramatic way, like through a mysterious portal. After that, think about the conflicts that will arise. For example, how will the wizarding community of that time react to Harry's strange ways? And don't forget to add some emotional moments, like when Harry and his grandfather have a heart - to - heart talk about family values.
Well, perhaps the grandfather could be teaching Harry all kinds of magic that his parents didn't have the chance to. He might introduce Harry to a different side of the wizarding world, full of old family secrets and forgotten spells.
There could be a 'Harry Raised Grandfather' fanfic where Harry is magically sent back in time due to a botched spell. He then finds his young grandfather in a difficult situation. Harry decides to take him under his wing. The fanfic could detail their relationship growth, like how Harry teaches his grandfather about the importance of love and friendship, which in turn shapes the grandfather into a different person, ultimately changing the course of the Potter family history.
One possible plot could be that his grandfather, perhaps a powerful but reclusive wizard, trains Harry in unique magic from a young age. Harry might grow up with a different set of values and skills compared to the original story. He could have a closer bond with the wizarding world's older traditions.
One possible plotline could be that Harry's grandfather imparts unique magical knowledge to him that he doesn't get at Hogwarts. Maybe it's an old - fashioned kind of magic that has been forgotten over time. This could give Harry an edge in dealing with the dark forces.
I'm not entirely sure as I haven't read this specific fanfic. But generally, if it's 'Harry raised by Gai', it might be a story where a character named Gai takes on the role of raising Harry in an alternate universe or fan - created scenario.
He might be more old - fashioned in his ways. His grandparents' values would likely have a big influence on him, so he could be more reserved and traditional compared to the Harry we know from the original series.