Hi Taoliver,
There are various different types of gel foams which you can see in post #2 here. Tempurpedic uses a very thin layer of gel that looks like it is “sprayed” on to their memory foam (#2 in the linked post) which would be among the least costly (and effective) methods of using gel compared to the amount of gel added in the other options.
Many (if not most) memory foam manufacturers have some version of gel memory foam mattresses that they have introduced in the last year or so that use gel foams with one or the other of the methods of adding gel to their foam (memory foam or otherwise).
Thermal conductive gel is one factor out of many that can contribute to the sleeping temperature of a mattress (and they are more effective when you are first on the mattress when there is a bigger temperature difference between you and the mattress and less effective when temperatures have evened out). You can read more about the many different factors that contribute to sleeping temperatures in post #16 here.
Phoenix