How do I reverse the slope of my garage floor?
Install a drain at the low spot, or rent a concrete saw and cut drainage channels (run the saw on tapered shims so the channels slope correctly.)
By cutting channels you should be able to intercept and redirect much of the water before it reaches the low spot.
It might be possible to do that to the whole floor with a terrazzo grinder, but those seem to be a lot harder to find for rental, and it would be a LOT more work. I doubt most things you'd apply on top would hold up to vehicle traffic unless you have room for a couple of inches at the thin part.