First off, a “Ghost Gun” is a firearm, usually home made, that lacks serial numbers.
3D Printing is nothing to do with it, any more than a hacksaw is. Back in the last century these were called “Zip guns,” made from pipe and rubber bands, and as much danger to the holder as anyone else.
A Briton by the name of P. A. Luty literally wrote the books on home made firearms. His designs cost less than £300 and don’t require anything more than ordinary DIY tools. His publications are widely available, and have been for decades. It’s not rocket science.
Most “3D printed” firearms seen in the USA rely on precision factory metal components (barrels, magazines, locking mechanisms etc.) which are not restricted over there.
It is interesting that these designs are often semi-automatic. It is considerably harder to make a semi-automatic gun than a fully automatic one unless you have access to precision parts. You’d think that hardened criminals would go for rapid fire, so frankly I suspect someone is just trying to show off.
AR’s, AK’s and so forth rely on the strength and wear resistance of metals to contain pressures measured in tens of thousands of PSI. In theory a plastic gun might hold a .22 round, but a plastic AR or AK is fantasy. Making parts other than the critical (and restricted) barrel and bolt is relatively simple using pretty much any process – even duct tape would suffice to hold them together.
There are designs that use 3D printed bits as templates and for low-stress parts, and common hardware store items for the stronger bits. But basically the same thing can be accomplished with printed paper templates, and aluminium or plywood. It’s simply a question of design, and by necessity these designs are not copies of normal firearms though they may superficially resemble them.
What’s missing here is 3D printed ammunition. That, I suspect, is a somewhat tougher nut to crack.
It has always been possible for malcontents to make their own functional firearms. They’re simple technology for anyone who can understand how to service a motorbike engine, for example.