This Guys Portal Gun Actually Works, Turns His House Into a Test Chamber. Not necessarily, once he flew into the air all of his earlier momentum was slowed, then halted, by gravity, then it was simply a matter of falling from that height... he would have gone very high up, but on his way down I don't think he would reach terminal velocity again (napkin math tells me it would be less, but I don't know how much less).
Even if it were substantially less, it *would* have killed him anyway, *if* all of that force was directed toward the Earth when he landed, but as soon as he went through the last portal, all of that gravity was transferred into horizontal force, and (like a bullet) he would have fallen to Earth no faster than if you dropped a baseball. That said, in the video it looks like he slammed into that fence, so... The solution would've been to put two portals on the ground and wait for gravity to slow his velocity as he fell up and down through both.
Of course, I imagine that the shifting gravity would cause you to vomit. OK Go - This Too Shall Pass - Rube Goldberg Machine version - Official. COOL TRANSFORMER COSTUME! How Long Can You Hold Your Breath.