In the swamp.
To see why Nomological is incompatible with Mary’s prerelease ignorance, it helps to first notice a very natural account that Nomological is able to give for knowledge of unobservables. Unobservable entities like electrons and radio waves are not detectable by the unaided senses. We may just as well state this fact by saying that there’s no law that directly relates sensory states to unobservables. However, some unobservables are detectable via the use of instruments. This later fact may just as well be stated by saying that there’s a law that relates instrument states to unobservables and another law that relates sensory states to instrument states. Electrons are not nomically related to tokenings of my electron concept in as direct a way as horses are to tokenings of horse. However, via mediating tokenings of instrument states, my electron concept is nonetheless psychosemantically grounded in electrons. Or at least, that is the story available to adherents of Nomological.
The trouble that arises for the gappy physicalists is that just as Mary may think about electrons that she’s never directly perceived, but detected via instrumentation, thus may Mary exploit Swamp Mary (or anyone else who knows what its like to see red) to think about a red quale. A person who has phenomenal knowledge is nomically related, so says Nomological, to red quale, and Mary, being physically omniscient, is nomically related to every physical state of a person who has phenomenal knowledge.
The trouble that Nomological presents to gappy physicalists is that Mary may utilize anyone who has phenomenal knowledge in much the same way as she uses measuring instruments. Prerelease Mary may even use Swamp Mary as an instrument.
It is no objection that Mary is in no position to herself calibrate such an “instrument.” Such an objection would say that since prerelease Mary doesn’t know what its like to see red, Mary is in no position to know that her instruments are detecting phenomenal redness instead of phenomenal greenness (or nothing at all). There are two main reasons why such an objection is quite poor. The first is that the objection simply begs the question against gappy physicalism, since non-gappy physicalists deny that prerelease Mary is phenomenally ignorant. The second reason is that it is quite natural to suppose that, for many of us, our capacity to think about unobservables is mediated by chains including instruments that we are not ourselves in a position to calibrate. I have no idea how to calibrate a machine for the detection of electrons. However, if Nomological is correct, then as long as some such device is so calibrated, it can serve as a link in the nomic chain relating my electron tokenings to electrons.