Not a major problem AND going after 3D printing doesn't actually solve the problem - the core components that make a gun a gun, such as the barrel, firing pin, etc., still need to be manufactured in the "traditional" way (unless you're trying to make a single use, one shot gun, but even that has better alternatives than 3D printing).
As it's been pointed out above, the pieces of equipment - lathes, mills, CNC machines - required to make the aforementioned parts require no licencing, no safety mechanisms to prevent gun part manufacturing.
A lathe takes no instructions and is potentially the most useful tool of all listed for the purposes of making a gun.