Hi Sallyb,
Welcome to the Mattress Forum! 
The gel that is used in various gel foams (most commonly memory foams) is a type of thermoplastic polyurethane that is used to change the performance characteristics of the foams. It is generally regarded to be less “toxic” than the polyurethane foam that surrounds it.
The gel is a response (among other responses and technologies both new and old) to two of several “issues” with memory foam IMO including responsiveness (ease of movement), temperature, the less supportive nature of memory foam (which is why it can’t be used effectively as a support layer), durability, and toxicity (there are food grade gels which are widely used including in the medical field). Since gel is more thermally conductive than other foam materials it feels cooler to the touch (which is great for marketing) and can also improve heat conduction away from the body to a degree (until the temperature equalizes). It can also improve the compression modulus and time dependent softening of memory foam which means that it can help prevent the tendency of memory foam to allow heavier areas of the body to sink down too deeply over the course of time. Compression modulus is the rate at which a material becomes firmer which is improved with gel. Viscoelastic materials get softer with temperature and humidity (which is more commonly known) but also have a property called “creep” which means that with constant pressure they also relax and get softer over time which is a less well known property of memory foam and gel can reduce this. Gel is also a less toxic material (uses “safer” chemicals and ingredients) and is very durable.
I have a good synopsis of how gels are used in memory foam mattresses in post #2 here and this post. There is also more detailed information in post #4 here about the technical nature of gel and toxicity questions.
Unfortunately, I can’t keep a record of the individual mattresses or their specs that the retailers and manufacturers in the hundreds of forum lists throughout the forum carry on their floor or have available online (it would be a bigger job than anyone could keep up with in a constantly changing market).
If you’re interested in looking online, you can use the expertise of the members listed in post #21 here who are all very experienced and knowledgeable and specialize in providing the type of help and guidance on the phone that can help you make good choices. There are a wide range of memory foam options included in the choices there (Rocky Mountain Mattress, Dreamfoam, Memory Foam Comfort, Nest Bedding, Novosbed, and Addable), and I believe that all of them compete well with the best in the industry in terms of their quality, value, service, and transparency. In addition to the online members of the site listed in the previous membership link … post #12 here has some of the better online memory foam options for those who are looking for a memory foam mattress. You may also wish to look at some of the memory foam offerings listed in the Simplified Choice thread here.
I hope that information is helpful to you.
Phoenix