Just to update this:
GCR as failed-clearcoat fix: As I stated in the first post, this did do something. I think a applied it again, it's a little complicated because this was failed clear on a back bumper cover, and I had tried some cheapo clear bra material on it a few years ago, which ultimately shriveled and peeled, so as more peels off I have applied some more GCR. Anyway, it seems to have helped some, which is better than nothing.
GCR as a tire coating: I actually haven't gotten a look at the tire back I pictured above, but I took the plunge and did the fronts of my daily-driver (as daily as it is in a pandemic), I dunno, about a month ago? I cleaned the tires with OPC and then some McKee's, and finally with some Tuf Shine cleaner. I haven't ever dressed the tires but they came with some greasy kid stuff from the dealer a year ago. Anyway, they came out great, too shiny for me at first, but they have toned down a bit. I don't know how the formula differs from the real Optimum tire coating, but this stuff seems fine, so far, oh, and none of the problems people have mentioned with trying to get an even finish with the tire coating, of course a lot of that has to do with the tire, age, previous dressings, cleaning, and, um...your level of perfectionism, which I certainly didn't have for this. All I know is they are going to be real easy to clean at the next wash.