Archive of Our Own is pretty much the undisputed king for this pairing, and honestly for most modern fandom fiction in general. The tagging system is a godsend for finding exactly what you're after, whether you want fluff, angst, or something darker exploring their complicated history. You can filter for word count, completion status, and tropes like 'alternate universe - coffee shop' or 'canon divergence', which is perfect because there are so many different interpretations of their relationship. I've found some incredible long-form fictions there that really delve into their dynamic pre-fallout, which is my personal favorite era to read about. The quality of writing tends to be higher than on more general sites, partly because the community norms encourage tagging and constructive feedback.
That said, I wouldn't completely write off fanfiction.net. Its interface feels ancient and searching is a pain, but it's got a deep archive, especially for older fics written while the manga was still serializing. Some real foundational takes on Gojo and Geto's bond are buried there, written before certain canon events were set in stone, and they have this fascinating speculative energy you don't see as much now. The downside is you have to wade through a lot more to find the gems, and the lack of nuanced tagging means you might stumble onto content you really didn't want to see.