There was no single inventor of the Little Boy gun bomb method,
as that method was an obvious means of assembling a supercritical
mass.
Seth Neddermeyer was the inventor of the implosion method for
the Fatman to assemble a supercritical mass fast enough to prevent
a fizzle when using reactor produced plutonium. However he was
unable to make it work and George Kistiakowsky was called in.
George Kistiakowsky invented the actual shaped charge explosive
lense system that was used in both the Gadget and the Fatman.