Well, this is bizarre. I deleted those saves, thinking they had corrupted color information or something, and re-played the ending of the game, with no cheat table file in my directory.
After I finished the finale, I came back into the simulation and looked in my garage (at that same gateway), and my cars looked just as chrome-glossy as before.
I load up a save before I start the finale, and my cars look normal.
I'm starting to get frustrated; this is making me feel like an idiot. I don't remember my cars ever looking this glossy (except the chrome vehicles), but I honestly haven't paid much attention to the weather as I've played through the game up to this point. Does the game go through day/night cycles before the finale or is it always eternal night, and therefore my cars only look weird now because up until this point they've had the same (dark) lighting conditions?
Edit: Yup, that seems to be it exactly. I assumed the game would always use the same lighting conditions when editing a car in the garage. But I changed the 'simulation override' to 'default' and my cars now look normal again.
But that means under normal daytime lighting conditions, gloss paint is REALLY GLOSSY.
But maybe that's why they did it that way, figuring that the only time you'd bother to use cars is in the beginning of the game when everything's eternal night, so that's when you'd want the gloss paint to look correct.
Well, I guess that solves that. Feel free to delete some or all of these posts, then.