Hi zman1,
There are many factors involved in temperature regulation besides just the foam in a mattress (see post #2 here) and I really doubt that anyone would be able to make a claim that their foam is the “coolest” unless they also released their comparisons with every other type of memory foam and the specifics of the testing method.
You can read a little more about the cooling effect of gel memory foam in post #8 here and the links it includes but in general it would depend on the specific formula of the gel foam. They will usually provide a cooler surface until temperatures equalize at which point they are more of an insulator (like regular memory foam) and it’s role in sleeping temperature would depend more on its airflow and the other temperature regulating factors of the sleeping system. I would take any claims of “this is the best” with a grain of salt unless it included the specifics of how it was tested and what it was being tested against.
Any waterproof protector will reduce airflow on the surface compared to a non waterproof more breathable protector and can increase sleeping temperature to some degree. Although it’s not enough to affect most people … it will affect some people who tend to be more sensitive or sleep hotter or where there are also other factors that can affect sleeping temperature.
Phoenix