First of all … on a subject that isn’t related to your question … I would suggest changing your username to something other than your email. There are many automated programs that scavenge email addresses that are listed in public to add them to spam lists. If you’d like me to change it to just “cliffcat” let me know through the contact link at the top of the page and I’ll change it for you.
On to your question …
The issue of the softness of memory foam (whether it is gel memory foam or regular memory foam) may be much more complex than you may realize. It is also a subjective perception that will be different to different people.
The softness of memory foam depends on heat, humidity levels, pressure, time on the mattress, and the age of the memory foam, and can vary depending on the conditions of its use. The same piece of memory foam may be softer or firmer depending on many variables not the least of which is the person on the mattress.
In addition to this … the thickness of memory foam and the layers both above and below it can have a major effect on its softness as well.
For example … if you had two mattresses with a very soft layer of memory foam that was 2" thick and this was over a firm foam in one and a softer foam in another (all memory foam mattresses use other materials under the memory foam because memory foam doesn’t have enough firmness or support to be used in the support layers) ) … then the mattress with the firmer foam under the memory foam would feel much firmer even though the memory foam layer was the same.
If you had two mattresses where there was a 2" layer of memory foam over a firmer layer of foam and another mattress that had a 3" layer of the same memory foam over the same firmer layer of foam below it … the thicker memory foam would feel much softer.
If you had a mattress that had a 3" layer of memory foam that was more temperature sensitive with a 1" layer of quilting foam or other quilting material on top of it … the warmth of your body wouldn’t reach it as easily as the same mattress without the quilting layer above the same memory foam which would feel softer.
If you had a mattress that used a woven cover that was less stretchy … it would feel firmer than a mattress that had identical materials that used a softer stretch knit cover.
These are just a few examples of how every layer and component in a mattress can affect every other layer.
Memory foams that are less temperature sensitive will also feel softer to some people but not to others. It can depend on how much you move because memory foam that is more temperature sensitive will feel firmer when you turn over onto parts of the layer that take time to soften even though it may get softer over a longer period when you aren’t moving.
There are hundreds of versions of memory foam (including gel memory foams) that are “rated” at various ILD’s (levels of softness) that will usually range from under 10 to the high teens (all of which would be considered to be in a range from super soft to soft in other materials) but these aren’t a reliable indicator of how soft the memory foam may feel to you. I also haven’t tested enough in side by side real time testing with all other conditions being equal (and I doubt that anyone really has) to know for sure which is the softest for me using a common thickness on the same additional layers and components much less which would feel softer to someone else. Even here … environmental conditions of heat, humidity, body weight, and others will affect which one feels softest in any particular set of circumstances.
In general though … lower density memory foams are less temperature sensitive (take less time to soften) and will feel softer to most people than higher density memory foams but the tradeoff is that they will also be less durable. Lower ILD numbers (most common in lower density memory foams) will also tend to feel softer than higher ILD memory foams but again … ILD can also be very misleading in memory foams.
The most reliable way to know how soft a particular memory foam mattress may be to you would be either by testing the mattress in person (which takes into account the effect of all the other layers and components) or by assessing all the different factors that affect softness and estimating which would feel softer to you “in theory” but this would not be nearly as accurate.
Because this can be amazingly complex and most consumers wouldn’t have the reference points or knowledge to know how to estimate this for certain (outside of personally testing a mattress for long enough to get a good sense of how the layering in a particular mattress felt to them) … the most effective way to estimate the softness of a particular memory foam mattress would be personal testing or conversations with a knowledgeable manufacturer or retailer who will tell you how their mattresses compare to other types and designs of memory foam mattresses.
Because they are so widespread … many manufacturers will also have some sense of how their mattresses may compare in terms of feel and softness to the Tempurpedic models which could be a helpful testing ground or reference point.
I would also keep in mind that while the softness (or actually the pressure relieving abilities) of a mattress are very important and involve more than just the memory foam layer or layers in the mattress … it is also important to make sure that the memory foam or other soft materials in the top layers of the mattress are not so soft and thick that you will be sleeping out of alignment where you may end up with back or joint pain or discomfort because of the tendency to choose a mattress that isn’t suitable for all your needs and preferences. Usually “just soft enough” for your sleeping positions that are more pressure sensitive (such as side sleeping) but no more is the best balance between comfort/pressure relief and support/alignment.
Almost every manufacturer who makes memory foam mattresses will have softer and firmer versions available that you can test in person. These will use variations in memory foam densities, types, and layer thicknesses along with firmer or softer transition and support layers and other components in the mattress.
The “trick” is to find the better factory direct manufacturers or better sleep shops in your area who have the knowledge and experience to help you make your best choices rather than trying to “match” the subjective memory of another mattress that you tested and are trying to “remember” every specific of how it felt over time, with movement, in different sleeping positions, and in different conditions.