Sounds very bizarre. I don't know the Pro-100, but the Pixma Pro9500 and Pro-1 that I've used are basically similar, pigment-based systems, and I have never seen any fading on any kind of paper - fine-art matte, matte, luster, glossy, or baryte type. (The baryte type papers mostly have an annoying metallic reflection in the shadows, but that's another matter.) Glossy might be a teeny bit darker in the shadows the moment it comes out of the printer, just because it's still a bit wet, but no there's no dramatic change when it dries.
Sounds to me you might have a bad batch of ink, or possibly a weird batch of paper that reacts strangely with the ink. (Or it might have something to do with the wrong black pigments being channeled for your glossy paper, though that would probably give you a print that looked wrong from the start. Likewise, a wrong color profile or bad color management.) Still, I don't see why anything should fade anywhere near this much.
good luck, Al