S&W:GB
I consider this a mediocre stand because it's physicality is garbage. It comes nowhere near close to stands like The World or King Crimson but it has an ability that transcends time hax such as those 2 stands and even RTZ.
There is a small possible chance Gappy explodes DIO's head with a Go Beyond Bullet even if time is stopped(GB sometimes works subconsciously) or manages to beat Diavolo due to the fact that his GB bullet would work even during the world of erased time.
For the rest, it's just Gappy potentially getting a window shot for GB and yes he's definitely beating Kira.
