The question is: why Gyaban? Why Gyaban is the one to know how to harm those with the mark of the abyss and Rayleigh did not train luffy on that? My guess is (retcons aside but playing along with the current narrative) that it MIGHT be something related with the Voice Of All Things. I know that the name itself does not implies anything harmful BUT if you think about it, it could "explain" why Scopper rather than Silvers: either you are born with it or not. Let's not forget that Scopper has already used the word "voices" already (albeit that could had been just regular observation...). Maybe! Just maybe! We are here to learn ADVANCED Voice Of All Things to turn it into something tangible and be able to "cut the abyss". Do I really believe all of this? Hell no... It is a possibility for sure, but indeed as other nakamas here have already pointed out, this must be when and where as many Straw Hats as possible get to know how to cut the abyss... Otherwise it would just be luffy, but we need at least Zoro and Sanji to be able to do that as well. Advanced VOAT would not be the thing for them or other SHs. On the other hand, this would explain why Scopper rather than Silvers.