Hi jbrill,
The “hand feel” of most gel memory foams will be noticeably cooler than regular memory foam but temperatures will tend to equalize over time at which point the insulating properties of the foam will become dominant. The amount and type of gel in the foam can affect whether the temperature benefits will last longer or shorter and the cell structure and amount of airflow through foam will also play a very significant role in reducing heat buildup in the material.
In other words (as you mentioned) … while gel memory foam can sleep a little cooler when you are first going to sleep at night … in most cases the benefits of the gel tend to be temporary and don’t normally last over the 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 sleeping temperature as the type of foam in a mattress) and on where you are in the “oven to iceberg” range (some people can sleep warmer on mattresses that most people are generally fine with) 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.
Your own careful testing or personal experience is the most reliable way to know whether any mattress is a good “match” for you in terms of comfort, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) but when you can’t test a mattress in person then the most reliable source of guidance is always a more detailed phone conversation with a knowledgeable and experienced retailer or manufacturer that has your best interests at heart and who can help “talk you through” the specifics of their mattresses and the properties and “feel” of the materials they are using (fast or slow response, resilience, firmness etc) and the options they have available that may be the best “match” for you based on the information you provide them, any local testing you have done or mattresses you have slept on and liked or other mattresses you are considering that they are familiar with, and the “averages” of other customers that are similar to you. They will know more about “matching” their specific mattress designs and firmness levels to different body types, sleeping positions, and preferences (or to other mattresses that they are familiar with) than anyone else.
Phoenix