I mean it depends, you could 3D print a lower receiver and fill in the rest with regular stock gun parts. The lower receiver basically holds ammo, and the trigger. In the case of a Glock for example it's the metal slide that does most of the work of actually shooting, the polymer frame is just there to hold everything in place.

This is why some guy was able to 3D print part of a Glock.

But is a gun hard to acquire? Than the skills required to 3d print and assemble one?

Depending on the country you're in yes, and on short notice even moreso.

As for assembling one, anybody who does proper gun maintenance takes their gun apart and puts it back together.

There's a market for DIY guns (outside of the 3D printing scene) where they ship you all the parts.

Here in Europe, definitely.

Europe is vague. In Norway it's definitely easier to buy one, than improvising this out of crap and sticks.

It's true for most of Europe though. Or - depends whether we're counting the black market.