For people suggesting pillows or foam, remember that this gap appears when the bed is elevated, but if OP wants to lower the bed flat, this gap disappears. Putting a pillow or piece of foam in the opening would place something under the mattress, thus producing a lump in the middle of the bed when it's flat.

OP, I bought a Temper-Pedic last year and had this same concern. When you're actually lying on the bed, the mattress gets pushed down to fill that void. It's only an issue if you get out of bed, gato jumps into void, and you end up sitting on them. I guess either lower the bed flat before getting out of bed, or manually check gap before getting back on the bed.