No, it's not that easy with plutonium; the design of plutonium device is not trivial. The nuclear characteristics of plutonium make it prone to pre-detonation (the chain reaction gets going before the optimum instant, with a decrease in the device's yield). The Manhattan Project had a great deal of difficulty in solving how to design the plutonium devices, which were used at Alomogordo and Nagasaki.
Was pre-detonation the reason that plutonium was deemed unsuitable for gun-type weapons?