Hi cbcb,
There are two processes that will happen together when you first buy a mattress. One of the adjustment period for your body to get used to a new sleeping surface that is different from what it is used to and the second is the initial break in period of the mattress which is when materials will settle, foams will lose some of their initial firmness, and the cover materials will stretch and lose some of their initial stiffness. both of these typically take up to 90 days or so (in rare cases even longer) but for most people it is less than 30 days.
The gel material itself won’t soften much at all but the other layers in the top few inches of your mattress (see the end of post #2 here) will go through some initial softening so it will end up feeling more like a floor model that has already been broken in.
Both your own “adjustment process” and the “break in” process will change how the mattress feels for you over the first few weeks of ownership but “how much” each person will feel both will vary with each person.
Phoenix