As I understand it... plutonium is a waste byproduct of light water and (esp.) breeder reactors. The primary challenge with plutonium is it cannot be processed like uranium, which which when bound to florine produces uranium hexaflouride gas suitable tor the centrifuge. The presence of plutonium240 interrupts the nuclear chain reaction because of high spontaneous fission rate (more neutron activity), thereby increasing risk of spontaneous detonation - a Bad Thing :-)
That's why Pu needs the implosion mechanism.
The Arabs/Koreans/other nuclear yahoos should be well-served with the U235 gun mechanism (and would of course be well-served by our assymetrical retaliation using U235 and Pu239-derived weapons).
The arab bomb design is not a gun mechanism.
It is an implosion device using U235.