What you are dealing with is a heavy clay topsoil. When this type soil dries out it shrinks. It’s because of this shrinkage that you have a gap along your foundation. The other clue in the photos is all of the cracking in the soil surface, another result of the shrinking.

I wouldn’t use bagged “topsoil” from a big box store to fill in the gap. It’s mostly compost and will remain quite permeable to moisture.

You are better off amending the soil in the area with organic matter and sand to break up the clay and make sure it’s graded to drain away from the foundation. Breaking up the clay should help the soil retain enough moisture and prevent shrinkage.