Actually plutonium is easier to detonate. The problem is that to avoid a "fizzle" a plutonium gun needs to bring its components together a lot faster and would have required a 30ft long bomb: too big for a B-29 bomb-bay, whereas the 10ft Little Boy fitted nicely.
Plus a gun requires 10X the explosive metal, and in '45 there wasn't enough plutonium for a single gun-device.
But U-235 can be used in implosion devices
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.
2. It's got to be big. Too big and heavy for a portable device, and very inconvenient for a clandestine one.
Not that big. Back c.1960 the size limit on implosion devices seemed to be >10" in diameter. So the first 8" tatical artillary shells were gun devices. maybe 300lb and 4ft long in the transport case, not lap-top size but reasonably portable.
Of course to go down to 155mm shell or 160mm mortar size, the weight of explosive metal gives a shell too heavy for the tube to fire, so you have to build a really tricky implosion device. Not for beginnners (see above re fears of South african tactical 155mm howitzers)
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.