Several corrections needed here. There's no such thing as a "plutonium gun" bomb. It is true that Pu239 needs to be assembled much faster, and that's precisely what makes an implosion design necessary. The Little Boy fit the B-29 nicely, but it was because they were able to reduce the size of the gun required below their initial estimates; in any event, it was a U235 device.
Plus a gun requires 10X the explosive metal, and in '45 there wasn't enough plutonium for a single gun-device.
It needs about 3 critical masses. Because the implosion design needs less than a critical mass (due to the compression of the material) the 10X figure is plausible.
It is true that in July '45 there wasn't enough Pu239 for a single gun-device there was never intended to be because they couldn't and weren't trying to make a Pu239 gun device; they were making an implosion device with it.
But U-235 can be used in implosion devices.
True, but irrelevant. If you can make U235, you make a gun device and obviate the technical challenges of an implosion device.
Incidentally the only post 45 nuclear power to go gun was South Africa, which lead to "informed opinion" considerable overestimating SA's capablity. The size of the reprocessing plant said SA could produce enough U-235 for 5-10 bombs a year, which lead to an estimated stockpile of 100+, 155mm tactical nuclear shells, neutron weapons and all the lumber of a major nuclear power. Real situation was just 6 quite heavy gun-devices.
And the reason they, or any nation, would go with a U235 gun design was to avoid the extraordinary expense of Pu239 breeder reactors, subsequent hot chemical processing, and the complexities of an implosion device.
And they pay for it with the need to make lots of U235 and in the jumbo size of the bombs that result.
It's true they couldn't make it, but early design work at Los Alamos until July 44 was on the Plutonium gun