So far, manufacturing your own gun in your workshop is not illegal under US legal code.
Making a reliable working gun is a different proposition, but instructions may be gleaned from any number of sources, readily and legally available. That genie is out of the bottle.
The 3-D “printable” gun is not a practical device without a few key elements being extensively modified, and such modifications would totally obliterate the “undetectable” nature of the weapon when concealed.