Not just that, but they use different types of nuclear fuel. The gun type used Uranium, which is very easy to set off. The implosion type uses Plutonium, which is harder to use... but WAY easier and cheaper to process.

Most of our weapons are Plutonium implosion devices, because Uranium is so difficult to enrich properly.

The first test bomb out in Trinity was a Plutonium implosion weapon, but the first weapon dropped was a Uranium weapon, little boy.

So they had no even tested it before that point, they tested it by actually using it on Hiroshima. They were so sure it would work after testing the much more complicated Plutonium implosion weapon that they didnt even bother.

The main reason we use Plutonium is that its easier to separate it. All the isotopes of Uranium weigh nearly identical amounts and have identical chemical properties, so its very difficult to separate out the useful Uranium from the useless Uranium.

Plutonium has a significantly different weight and chemical properties from Uranium, so it can be separated very easily. Making it vastly cheaper to produce than enriched Uranium.

Plutonium is actually a byproduct of using low grade Uranium in reactor. So its both easy to produce, and easy to separate.