Hi PatKauf,
I think that Jeff has covered most of the pertinent points in reply to your question but I’ll add a couple of comments as well.
Gel memory foams (and other types of gel foams) include a very wide range of different materials with different formulations so they aren’t just a single material with the same temperature regulating properties but there is more information about gel foams in general in post #2 here and the posts it links to. In very general terms … gel and/or other thermal conductive or phase change materials can have “some effect” on on the sleeping temperature of a particular material but how much of an effect they will have and how long the effect will last will depend on the specific formulation of the material and on the “combined effect” of all the other materials and components of the sleeping system including your sheets, mattress protector, and bedding. Gel materials tend to have a more temporary effect when you first go to sleep at night or over the first part of the night than they will over the entire course of the night.
While it’s not possible to quantify the sleeping temperature of a mattress for any particular person with any real accuracy because there are so many variables involved including the type of mattress protector and the sheets and bedding that you use (which in many cases can have just as significant an effect on temperature as the type of foam in a mattress) and on where you are in the “oven to iceberg” range and because there is no standardized testing for temperature regulation with different combinations of materials … there is more about the many variables that can affect the sleeping temperature of a mattress or sleeping system in post #2 here that can help you choose the types of materials and components that are most likely to keep you in a comfortable temperature range.
Phoenix