China's complaint is here. There's a challenge to the statute itself, as well as its application in the case at hand.
It's difficult to imagine that this one will get settled, so in all likelihood this is just step 1 on the way to a panel. It will be interesting to see if China elaborates on the claims any more in the panel request.
One thing I found interesting was the way the claim was presented:
China considers that these higher tariffs, not having been justified as emergency action under relevant WTO rules, are inconsistent with Article I:1 of the GATT 1994 because the US does not accord the same treatment it grants to passenger and light truck tires originating in other countries to the like products originating in China.
Nor can the US justify these measures as properly applied exceptions to this fundamental WTO principle. These measures have not been properly justified pursuant to Article XIX of GATT 1994 and the Agreement on Safeguards. Nor have these measures been properly justified as China-specific restrictions under the Protocol on the Accession of the People's Republic of China (the Protocol of Accession).
So instead of simply claiming a violation of the Protocol, China characterizes its claims as being under GATT Article I, and then says that no "exceptions" have been met. I should probably double check before I say this, but I don't feel like these kinds of complaints are usually presented this way. For example, a complaint under the Safeguards Agreement would just allege violations of the Safeguards Agreement, without getting into the relationship with GATT Article I. I've often wondered why complainants don't take the approach China has taken here. It remains to be seen, though, whether there is any benefit to this approach (e.g., in terms of burden of proof).
ADDED: Julia's comment on this post makes me realize I probably didn't get this quite right. With regard to the normal safeguard provisions, the issue may not be Article I (MFN) so much as Article II (tariff concessions) or Article XI (quotas). While it is possible that a Member might complain about MFN discrimination in relation to a safeguard measure, at least in theory these are global measures that apply in a non-discriminatory manner. (With anti-dumping, by contrast, both Article I and Article II could generally serve as the basis for a claim.)
