The Uranium gun device was simplicity itself, and the physics was well understood and quite tractable. The problem with plutonium is that the reaction is so fast that the bomb would tear itself apart before much of the plutonium can fission in such a device. The implosion device depends on an array of explosive charges surrounding a plutonium sphere need to be triggered to all explode at once, within less than a microsecond of one-another, and impart a near-perfectly symmetrical shockwave to compress the sphere enough that it goes critical, but only for about one microsecond. Only about two-percent of plutonium actually fissions, but, man does it do a job. The energy released by a fissioning nuclide is about one-million times greater than the same mass of TNT.
There's no problem with a plutonium gun bomb. In fact that's how the Thin Man bomb was designed.
The problem wasn't the design, the problem was that they had done their test measurements (which led to the design) using minute amounts of high-purity Pu-239 produced by Oak Ridge's cyclotrons. But to get the mass they needed for the bomb, they had to use plutonium made in the atomic reactors in Washington.
The problem with the reactor plutonium was it had a higher concentration of Pu-240 contamination than the Oak Ridge plutonium. And Pu-240 has a much higher rate of spontaneous fission than Pu-239, meaning it would have initiated the reaction before the two fissile masses had come together, producing a fizzle rather than a bang.
They could have overcome this with by increasing the speed of the gun but they already were at he pressure limits of the gun's barrel so the only way to increase velocity would have been with a longer barrel. But to get the velocity they needed the barrel would have been longer than could be transported by any bomber they then had, which left the implosion bomb, which was less fussy about Pu-240 levels, as the only alternative.