I believe you are describing a plutonium implosion device not a u-235 device. U-235 is hard to refine but easy to detonate (one of the reasons the little boy design was never tested). Plutonium is easier to refine but harded to detonate. (we tested the fat man design before using it).
Actually the implosion device can be used for either.
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)