Hi Harriett,
You can read a little more about gel foams in post #2 here. How long ther cooling effect lasts will depend on the percentage of gel in the material but for the most part they will have a temporary effect when you are first going to sleep at night but not over the whole course of the night.
They will still have some effect with a mattress protector and your sheets and bedding but the effect will be reduced with thicker protectors and/or sheets.
The better sources for mattress covers I’m aware of are listed in the component post here but I don’t know of any that include gel or phase change materials that are available separately.
Phoenix