The news that Hiroshi put him there is the least surprising of what he says. The silver eyes are a sign of his immortality. Further, he can go 4 minutes without oxygen, 4 days without water, and 40 days without food - not spectacularly better than what we humans can do, but definitely an improvement in our survival quotients. There is also a limit of 500 on how many immortals there can be, presumably at any one time.
Most interesting and profound of what the immortal in the basement says is what a drag it is to be immortal. The action of the genes which compels the continuation of life is no fun to feel. His price for immortality is ten thousand or whatever deaths. This part of Helix thus puts it in league with a splinter of a genre in science fiction, in which immortality proves to be a curse. I recall a story I read years ago, in which the immortal just got gradually older, more and more decrepit, wasting away, but never dying. In this context, what Gunnar wants most is to be put out of his immortality.
Other facts that come to light in Helix 1.10 is that the late Constance Sutton's group all have silver eyes - we already knew that Constance had them - and are far more than a greedy, ruthless corporation. But still unknown is whether they are aliens, or humans who have evolved to a higher level (though not higher as far as the guy in the basement is concerned).
Meanwhile, back at the base, Sarah's cancer has progressed. But, as was apparent and I've been saying since we saw Sarah's first tremor, some combination of the virus and bright eyes will save her.
Helix is still taking its time to reveal what's going on, but is now spiraling along quite well.