The best the government might do, short of banning what is heralded as the biggest thing for consumers in the next decade by some, is to force 3D printers to embed the serial number of the device into what they produce, but that alone might interfere with the structural integrity of the object.
The whole concept is moot though as if one were a criminal, you could buy a gun on the street cheaper than making one. Take one from someone in a secured area easier than smuggling one in is another option. I recall as well that organized crime ran an operation at a New Jersey plant where they used the lighter staffed second and third shift to manufacture firearms by resetting the tooling then setting it back, so that first shift had no clue. It ran for at least five years according to law enforcement. So organized crime will just manufacture guns. Lower echelons will steal them or buy them from people who stole them. Or legitimately purchase them. Or pay someone to purchase them for them.
Hobbyist types will continue to make them. 3D printing is just one technology. For the price of the higher quality 3D printer used to make the functional 3D firearm (this was not RepRap technology!) one could purchase a nice combo lathe and mill, and the CNC kit for it. Then produce the parts in metal. The afghani people have used hand tools and sourced their raw materials from things like car axels and scrap sheet steel and used very minimal machine shop work (I presume to machine the barrel / bore it out / chamber it) and the rest by hand with drills and files and a smallish box and pan brake. They cranked out enough AK replicas that they used a 55 gallon drum of boiling "paint" to coat them with (after care to protect the barrel and friction surfaces). A 3D printer (in plastics) just makes it simpler to make a bad gun. And is very expensive as well in comparison to making one out of metal.
The purpose of the 3D printed gun was to underline dramatically how futile it is to enact gun control in the manner the US tries to do. Not to produce marketable or truly useful completely plastic devices. And it did that to great excess. And as to the undetectable nature. I could as easily CNC machine plastics or even unfired ceramics to the same end. Heck, some materials I might be able to cast from easy to make molds.
Lastly if one has a legitimate firearm, and one wants to make a replacement part for that firearm, then that should be allowed. Do I insert a FOID card into the machine to allow it (assuming the impossible technology to detect it is destined for a firearm and not a toaster or my car stereo)? This is not a tractable problem. This poll is selective and designed to get the response reported