Hi bwomp,
Temperature regulation can depend on many factors and sometimes even a simple change such as a different mattress protector or set of sheets can make a significant difference. Temperature regulation in other words is a combination of influences (including of course the mattress and the person themselves).
The gel memory foams can be one of the factors that help to make a mattress cooler for some people (at least until the temperatures equalize). There are some thoughts on all the different factors that can contribute to sleeping temperature in post #2 here to give you a sense of some of the things that can play a role both in the mattress and in the bedding you choose.
I think that one of the reasons that you find that mattresses sleep hot for one person and not another besides of course the people themselves is because of the many different factors that can be involved in humidity control and temperature regulation and in some cases people think it’s only the mattress when it may be some of the other “layers” that they can change (although of course the types of foam and the mattress ticking and quilting also plays a major role as well).
Phoenix