There are a number of incorrect claims wandering around. Rather than answer each post, I'll summarize.
Like most machines, a gun is a collection of parts. For various reasons, one of those parts must (legally) be the gun itself, and the rest are just parts attached to the gun. For guns similar to (or clones of) the AR-15/M-16, the gun is the lower receiver.
The other parts are not restricted at all. Anyone can buy barrels, springs, sears, stocks, triggers, hammers, whatever they want, off the street, over the internet, or mail order. No ID, no registration.
If a dealer is selling a receiver, either alone, or as part of a completed firearm, they have to do the background check, fill out the paperwork, etc. A non-dealer doesn't need to do any of that, but the ATF will consider you to be a dealer if you act like a dealer.
The receiver is a complicated part. It takes a lot of work to turn a piece of metal into a receiver. At some point during that work, it changes from "piece of metal with some cuts" into a receiver.
Pieces of metal that have had some work done, but not enough to become receivers, are sold as "80% receivers". These are subject to no more regulation than any other random block of metal, because it is the end user that actually manufactures the gun.
Building your own gun is perfectly legal, by the way, as long as you are doing it for yourself. If you intend to sell it, or give it away, you need to get licensed and pay for a tax stamp. If you decide later to sell it, or give it away, that is perfectly legal too, but you need to make sure that you don't do anything that would make a reasonable person think that you had intended to pass it on when you made it.
The Ghost Gunner ONLY works on these 80% Receivers. They are not capable of milling a receiver out of raw billet. Nor could they work with a raw casting or forging.
Desktop milling machines don't have the power to spin up a heavy chuck, nor, generally, could they manage enough axis velocity to keep the feed rate up when using a large diameter tool. That means 1/8" or 1/4" chucks and tools. That limits the milling depth two an inch or two. That's plenty for milling out the trigger pocket, but nowhere near enough for the magazine well.
And if anyone is interested in the topic, there is a forum thread somewhere showing a guy making an AK receiver out of a shovel. The same technique has been used around the world. The Afghans made their AKs in caves, with hand tools.