I guess it all comes down to the very vague and wide-open Part A (bolded by me):
It shall be unlawful for any individual knowingly to possess a firearm that has moved in or that otherwise affects interstate or foreign commerce at a place that the individual knows, or has reasonable cause to believe, is a school zone.
Since the initial GFZA that was passed was ruled unconstitutional and they changed it to specify it only pertains to firearms that have been active in interstate or foreign commerce. most of the circuit courts have upheld the revised version.
So, ultimately it comes down to how does one define whether a firearm has "moved in or otherwise affects interstate or foreign commerce". That is an incredibly broad statement, and given the very wide-open interpretation of the Commerce Clause and the various presidence-setting rulings by the Supreme Court, I think one would have a very difficult time arguing that even a firearm manufactured solely in washington does not "move in or otherwise affect interstate or foreign commerce". When the Court has ruled (in Wickard v. Filburn) that growing wheat on one's own land purely for one's own consumption is something congress can legislate under the Commerce Clause because the whole of a bunch of individual farmers growing their own wheat could potentially impact interstate commerce...
I imagine they would also make the argument that the materials used to manufacture the firearm were not 100% sourced from Washington so purchasing those materials affected interstate or foreign commerce, or that the machines that were used to manufacture the firearm did, or that any number of other justifications make it so that that firearm in some way (directly or indirectly) affects interstate or foreign commerce.