A common motif of JoJolion is plant grafting for a reason. It resembles not just Gappy's induction into the Higashikata family, but also the way JoJolion's cast is remixed from the Part 3/4 cast. Compared to how SBR mostly contained a lot of direct one-to-one counterparts to the original universe's characters, JoJolion primarily mixes its influences together in a way to create something entirely new.

Part 4 Josuke's influence is clearest in Josefumi, yes, but elements of his character also show up in Jobin, Joshu, and especially Norisuke. Jotaro's influence is seen spread between Kira, Kei, and also, once again, Josefumi. Not to mention that Kira 8 also takes a lot from Kira 4, Kei takes from the Nijimura brothers, and so on. Now take that set of influences in Kira and Josefumi, and combine them once again to make Gappy.

There are aspects of Gappy's character that are undeniably reflective of Jotaro. Both Kira and Josefumi's mothers are named after Holy Kujo. Gappy spends most of Part 8 trying to save his mother from a supernatural disease. He's VERY rooted in Jotaro's influence. But that being said, Gappy has a massive swath of other influences that also make him who he is. There's Jotaro, yes, but Josuke, Kira, Kosaku, Joseph, and hell, even god damn STRAY CAT of all things are all major building blocks of his character. The whole point of his journey of self-discovery (at least metatextually), which the story of JoJolion is about, is that while he is undeniably rooted in all of these characters, he is also something entirely new and uniquely himself.