Plutonium cannot be used in gun-type bombs because of P-240 contamination, but the basics are still the same. If you do not get the U-235 into the correct conf

Plutonium cannot be used in gun-type bombs because of P-240 contamination, but the basics are still the same. If you do not get the U-235 into the correct configuration quickly enough you will have a fizzle. The difference between plutonium and uranium in a gun type bomb is that there really isn't a practical "quickly enough" with plutonium. The more pure you get your U-235 the more you can reduce the amount of time that you have to get it into a proper configuration. Pure enough to just drop together though? I find that incredibly unlikely. Keep in mind that even with incredibly pure U-235 your fuel is going to start to become critical when the two halves are still a few inches apart. Once that happens you have a very short period of time to close the gap and 9.8m/s^2 is pretty shoddy. The two halves are also quite massive, so being strong enough to slam them together by hand fast enough is not really reasonable either, they have too much inertia. Maybe you could make a gun-type bomb using gravity if you dropped one half down a long evacuated tube... easier just to make it properly though. Of course refining pure enough U-235 that "simpler" gun designs become possible would be more difficult than actually just building the rest of the bomb correctly anyway. I think Luis Alvarez was exaggerating "high yield" (you could certainly create a very damaging fizzle), or more likely "dropping", since constructing the gun is rather trivial with highly pure U-235.