Hi Fluffy,
The Serta iComfort uses about 5lb gel memory foam based on my calculations (since they don’t release this information specifically since it would be too easy for people to make meaningful comparisons which would lead to the recognition of its poor value).
For some people … the gel may make a small difference in heat, however it is not likely that it will make as much difference as a more open celled foam such as Aerus or even memory foam which is ventilated with holes in it. Memory foam in general though will sleep hotter than other foams because it is less open celled and you will sink in deeper than other foams. A cover made of some of the newer materials which helps regulate heat can make some difference here as well and so can a wool mattress pad (although the tradeoff is that the pad will lower the ability of the memory foam to “melt” and conform to your body shape and it will feel firmer). In general though memory foam is still the hottest of the foams which for some is an issue … and for some less so.
There is a lot of research into many different types of “gel” additives being added to memory foam in recognition of some of the drawbacks of memory foam itself which is now becoming more well known as consumers are becoming a little more aware. A good article about this is here http://www.furnituretoday.com/article/542466-Gel_Beds_in_Spotlight.php
As you can see … many manufacturers are trying to play catch up with a technology which I believe may have some limited benefits (in some specific instances) but overall will do more to confuse people. We are about to become inundated with “competing claims” and stories about the “benefits of gel memory foam” of various types and most of them will be just that … stories … meant to sell products without any real factual information behind them and with a goal of improving profits and market share. They are based on “perception” much more than fact. Much of this IMO is being driven by Tempurpedic’s increasing market share through their own “branding” information and advertising which is taking advantage of the general state of confusion and misinformation in the memory foam segment of the industry. This too is unfortunate as there are certainly memory foams which are as high quality as Tempur at a much lower cost but do not have the same brand recognition.
I wrote a reply just before this one which talked about the comfort and support functions and layers of a mattress which are the two main functions of a mattress. It is in reply #6 in this thread. It links to a couple of articles on the site which should be helpful. There is also an article about memory foam here which talks about memory foam in general and how important the support layers under the memory foam are since memory foam itself is the least supportive of all foams and needs a supportive layer underneath it. There is an even more detailed memory foam article here.
And by all means keep any questions coming. That’s why I’m here 
Phoenix