The element indicates the value for the "Gradient Factor Low" (GF Low), given as a real number 0.0 <= GF Low <= GF High <= 1.0. For further explanation see Clearing Up The Confusion About "Deep Stops" by Erik C. Baker, e.g.
<gradientfactorlow> should be less than <gradientfactorhigh>.
<decomodel>, <gradientfactor>, <gradientfactorhigh>, <tissue>