You mean "gun-assembled." The only gun assembled device ever used was Little Boy on Hiroshima. Fat Man was an implosion type device and the trinity device was also implosion.
Our first use was Gun assembled because the men who designed it knew it would work and thus wanted to make sure the first "demonstration" worked. Nagasaki was to make sure the Trinity type device worked in a operational environment and since Hiroshima was already leveled a fizzle would not have been quite as embarrassing.
That is exactly what I meant - I was obfuscating, you know?