Lets imagine a similar situation but instead of with an additive manufacturing process they try to regulate a subtractive manufacturing process: a traditional CNC machine. There is no way to prevent the CNC system from machining gun parts as along as the machining is done in discrete steps with the same work piece. The software can't know what sitting on the CNC table.

In additive manufacturing it is more difficult but not impossible to print a bunch of pieces that look nothing like a gun part but and in the end be assembled into a gun.

In both the above cases there would need to be sophisticated surveillance software to even come close to detecting "gun-ness."

While I don't have a horse in the gun control race, I do have one in the open-source, running a local OS, running what software I want, and controlling what that software does races.