Does this make GPL unenforceable, or am I reading too much into this?
It doesn't read like it would prevent an individual from applying the GPL to their software but it might prevent the government from doing the same... Which would be exceptionally strange and problematic.
It could mean that government agencies would be prohibited from contributing to GPL-styled open source software or even using said software since they would be required to distribute it to anyone that asks for the code.
So for example the Japanese government cannot force a US company to release source code as a condition to allow its use/import/sell in Japan. But the Japanese government can write/finance as much GPL software as it wants, and can import as much GPL software as it wants. The Japanese government can even mandate all software for its own procurements to be GPL-only.
What Japan cannot do is outlaw the sell or use of closed source foreign (by a person of another Party) software on its territory. So say if Ford wants to sell cars in Japan, the Japanese government cannot force them to release the source code for a mandatory code-audit as a condition for import...
I really don't think it's a good article.
(IANAL, of course)
It has a bunch of vague hooey about the clause being limited to "mass market software" which I think is designed to deal with that. Of course, it's pathetically poorly defined (if a car is sold to a "mass market" then it's software is too, right?). To me this clause was pretty much written by Microsoft and of course, the negotiators at the TPP table are far to clueless to understand any of the subtlety involved in software licensing and distribution.
Imagine that Google launches a self driving car in Japan, but the Japanese government ask access to its source code... How many hours before Toyota gets it and start "auditing" it?
Why would that be a problem? Sounds perfectly reasonable to me as long as everyone has the right to audit the code. There's a huge public interest in disclosure of source code for things like self-driving cars.
If Toyota, Mitsubishi, and other Japanese companies were given the code but no one else then there would be a problem.
The big problem is the use of "require", because it's a very loose general term. It doesn't matter what basis the "requirement" comes, so even just ordinary old copyright law seems to come into scope there.
I don't think it's a real problem but I do think it is close enough to one that it's a horrible thing to have in the agreement.
EDIT: further to those thoughts, the result would not be that the GPL is unenforceable, rather, the result would be that people would lose the right to distribute GPL software in the country. A working GPL is the only thing that gives them the right to do that in the first place. I doubt there is any nation left on earth that can actually now operate without access to GPL software, this is unlikely to play out even if there is a way to interpret it that seems to violate the GPL.
Governments are not territories: "No Party shall require [...] access to source code [...] as a condition for the import [...] in its territory.". Because a given closed-source software cannot be bought in a specific procurement doesn't mean that it cannot be sold/used in the territory, so the text doesn't apply.