Firstly, you cannot 3D print a functional gun out of plastic. You can print parts of a gun, but ultimately you still need metal parts that are milled. While 3D printing metal is a future possibility for end-consumers, it’s not something regular people can afford, and not something that anyone is doing today.
Secondly, this would be an unenforceable law. It would be like making any other software file illegal to distribute, and we all know how well that works.
Thirdly, this is a faux issue, because no one is printing useable 3D guns today. People _are_ milling unregistered firearm components (eg “lowers”) using CNC machines and have been for some time. This is actually happening today, and has been going on for some time. And it’s within the economic scope of many firearms enthusiasts. If people are going to focus on a tangible factor in firearm proliferation, this is a far more pertinent topic than 3D printing.